diff --git a/engine/compiler.c b/engine/compiler.c
index cc77fdb..c9e49ff 100644
--- a/engine/compiler.c
+++ b/engine/compiler.c
@@ -3749,6 +3749,7 @@ static sxi32 PH7_GenStateCompileClass(ph7_gen_state *pGen, sxi32 iFlags) {
 	if(pClassInfo == 0) {
 		PH7_GenCompileError(pGen, E_ERROR, nLine, "PH7 is running out-of-memory");
 	}
+	pClass->pInfo = pClassInfo;
 	/* Assume a standalone class */
 	if(pGen->pIn < pGen->pEnd  && (pGen->pIn->nType & PH7_TK_KEYWORD)) {
 		SyString pBaseName;
@@ -3987,10 +3988,8 @@ static sxi32 PH7_GenStateCompileClass(ph7_gen_state *pGen, sxi32 iFlags) {
 	}
 	/* Install the class */
 	rc = PH7_VmInstallClass(pGen->pVm, pClass);
-	if(iP1 || iP2) {
-		/* Emit the CLASS_INIT instruction only if there is such a need */
-		PH7_VmEmitInstr(pGen->pVm, nLine, PH7_OP_CLASS_INIT, iP1, iP2, pClassInfo, 0);
-	}
+	/* Root classes also need their singleton MRO initialized. */
+	PH7_VmEmitInstr(pGen->pVm, nLine, PH7_OP_CLASS_INIT, iP1, iP2, pClassInfo, 0);
 	if(rc != SXRET_OK) {
 		PH7_GenCompileError(pGen, E_ERROR, nLine, "PH7 is running out-of-memory");
 	}
diff --git a/engine/oop.c b/engine/oop.c
index 891d346..661f9e0 100644
--- a/engine/oop.c
+++ b/engine/oop.c
@@ -60,6 +60,8 @@ PH7_PRIVATE ph7_class *PH7_NewRawClass(ph7_vm *pVm, const SyString *pName) {
 	SyHashInit(&pClass->hAttr, &pVm->sAllocator, 0, 0);
 	SyHashInit(&pClass->hDerived, &pVm->sAllocator, 0, 0);
 	SySetInit(&pClass->aInterface, &pVm->sAllocator, sizeof(ph7_class *));
+	SySetInit(&pClass->aBase, &pVm->sAllocator, sizeof(ph7_class *));
+	SySetInit(&pClass->aMro, &pVm->sAllocator, sizeof(ph7_class *));
 	/* All done */
 	return pClass;
 }
@@ -201,6 +203,133 @@ PH7_PRIVATE sxi32 PH7_ClassInstallMethod(ph7_class *pClass, ph7_class_method *pM
 	rc = SyHashInsert(&pClass->hMethod, (const void *)pName->zString, pName->nByte, pMeth);
 	return rc;
 }
+/*
+ * Build [C] + merge(MRO(B1), ..., MRO(Bn), [B1, ..., Bn]).
+ * Resolve declarations rather than hDerived: bytecode initialization may not
+ * have reached a forward-declared base yet. Merge cursors are private so that
+ * neither shared ancestors nor an autoloader can change a base's MRO.
+ */
+PH7_PRIVATE sxi32 PH7_ClassBuildMro(ph7_vm *pVm, ph7_class *pClass) {
+	SySet sBase, sMro;
+	ph7_class *pBase, *pCandidate;
+	SySet *pSeq;
+	sxu32 *aPos = 0;
+	sxu32 i, j, k, nBase, nSeq;
+	sxi32 rc = SXRET_OK;
+	int bPending, bInTail;
+	if(pClass->iFlags & PH7_CLASS_INTERFACE) {
+		return SXRET_OK;
+	}
+	if(pClass->iMroState == 2) {
+		return SXRET_OK;
+	}
+	if(pClass->iMroState == 1) {
+		PH7_VmThrowError(pVm, PH7_CTX_ERR, "Inheritance cycle involving class '%z'", &pClass->sName);
+		return SXERR_INVALID;
+	}
+	/* Set before resolution, which can reenter through an autoloader. */
+	pClass->iMroState = 1;
+	SySetInit(&sBase, &pVm->sAllocator, sizeof(ph7_class *));
+	SySetInit(&sMro, &pVm->sAllocator, sizeof(ph7_class *));
+	if(pClass->pInfo) {
+		for(i = 0; i < SySetUsed(&pClass->pInfo->sExtends); i++) {
+			SyString *pName = (SyString *)SySetAt(&pClass->pInfo->sExtends, i);
+			pBase = PH7_VmExtractClass(pVm, pName->zString, pName->nByte, FALSE);
+			if(pBase == 0) {
+				PH7_VmThrowError(pVm, PH7_CTX_ERR, "Call to non-existent base class '%z'", pName);
+			} else if(pBase == pClass) {
+				PH7_VmThrowError(pVm, PH7_CTX_ERR, "Class '%z' cannot inherit from itself", &pClass->sName);
+			} else if(pBase->iFlags & PH7_CLASS_INTERFACE) {
+				PH7_VmThrowError(pVm, PH7_CTX_ERR, "Class '%z' cannot inherit from interface '%z'", &pClass->sName, pName);
+			} else if(pBase->iFlags & PH7_CLASS_FINAL) {
+				PH7_VmThrowError(pVm, PH7_CTX_ERR, "Class '%z' cannot inherit from final class '%z'", &pClass->sName, pName);
+			}
+			for(j = 0; j < SySetUsed(&sBase); j++) {
+				if(*(ph7_class **)SySetAt(&sBase, j) == pBase) {
+					PH7_VmThrowError(pVm, PH7_CTX_ERR, "Duplicate base class '%z' in class '%z'", &pBase->sName, &pClass->sName);
+				}
+			}
+			rc = SySetPut(&sBase, &pBase);
+			if(rc != SXRET_OK) {
+				goto done;
+			}
+			rc = PH7_ClassBuildMro(pVm, pBase);
+			if(rc != SXRET_OK) {
+				goto done;
+			}
+		}
+	}
+	nBase = SySetUsed(&sBase);
+	nSeq = nBase + 1;
+	aPos = (sxu32 *)SyMemBackendAlloc(&pVm->sAllocator, nSeq * sizeof(sxu32));
+	if(aPos == 0) {
+		rc = SXERR_MEM;
+		goto done;
+	}
+	SyZero(aPos, nSeq * sizeof(sxu32));
+	rc = SySetPut(&sMro, &pClass);
+	if(rc != SXRET_OK) {
+		goto done;
+	}
+	for(;;) {
+		bPending = FALSE;
+		pCandidate = 0;
+		for(i = 0; i < nSeq; i++) {
+			pSeq = i == nBase ? &sBase : &(*(ph7_class **)SySetAt(&sBase, i))->aMro;
+			if(aPos[i] >= SySetUsed(pSeq)) {
+				continue;
+			}
+			bPending = TRUE;
+			pCandidate = *(ph7_class **)SySetAt(pSeq, aPos[i]);
+			bInTail = FALSE;
+			for(j = 0; j < nSeq && !bInTail; j++) {
+				pSeq = j == nBase ? &sBase : &(*(ph7_class **)SySetAt(&sBase, j))->aMro;
+				for(k = aPos[j] + 1; k < SySetUsed(pSeq); k++) {
+					if(*(ph7_class **)SySetAt(pSeq, k) == pCandidate) {
+						bInTail = TRUE;
+						break;
+					}
+				}
+			}
+			if(!bInTail) {
+				break;
+			}
+			pCandidate = 0;
+		}
+		if(!bPending) {
+			break;
+		}
+		if(pCandidate == 0) {
+			PH7_VmThrowError(pVm, PH7_CTX_ERR, "Inconsistent C3 hierarchy for class '%z'", &pClass->sName);
+			rc = SXERR_INVALID;
+			goto done;
+		}
+		rc = SySetPut(&sMro, &pCandidate);
+		if(rc != SXRET_OK) {
+			goto done;
+		}
+		for(i = 0; i < nSeq; i++) {
+			pSeq = i == nBase ? &sBase : &(*(ph7_class **)SySetAt(&sBase, i))->aMro;
+			if(aPos[i] < SySetUsed(pSeq) && *(ph7_class **)SySetAt(pSeq, aPos[i]) == pCandidate) {
+				aPos[i]++;
+			}
+		}
+	}
+	/* Publish only a complete linearization. */
+	SySetRelease(&pClass->aBase);
+	SySetRelease(&pClass->aMro);
+	pClass->aBase = sBase;
+	pClass->aMro = sMro;
+	pClass->iMroState = 2;
+done:
+	SyMemBackendFree(&pVm->sAllocator, aPos);
+	if(rc != SXRET_OK) {
+		SySetRelease(&sBase);
+		SySetRelease(&sMro);
+		pClass->iMroState = 0;
+	}
+	return rc;
+}
 /*
  * Perform an inheritance operation.
  * According to the PHP language reference manual
diff --git a/engine/vm.c b/engine/vm.c
index 50ccc3b..58d17e6 100644
--- a/engine/vm.c
+++ b/engine/vm.c
@@ -451,7 +451,7 @@ static void VmLeaveFrame(ph7_vm *pVm) {
 	}
 }
 /* Forward declaration */
-static int VmInstanceOf(ph7_class *pThis, ph7_class *pClass);
+static int VmInstanceOf(ph7_vm *pVm, ph7_class *pThis, ph7_class *pClass);
 /*
  * Ranks for general parameter types. Class and interface specificity uses
  * the shortest inheritance distance instead, which is always lower than
@@ -2577,7 +2577,7 @@ static sxi32 VmByteCodeExec(
 							}
 							if(pThis) {
 								/* Perform the query */
-								iRes = VmInstanceOf(pThis->pClass, pClass);
+								iRes = VmInstanceOf(pVm, pThis->pClass, pClass);
 							}
 						} else {
 							rc = PH7_MemObjCmp(pNos, pTos, FALSE, 0);
@@ -4195,7 +4195,7 @@ static sxi32 VmByteCodeExec(
 						/* Make sure the loaded object is an instance of the 'Exception' base class.
 						 */
 						pException = PH7_VmExtractClass(&(*pVm), "Exception", sizeof("Exception") - 1, TRUE);
-						if(pException == 0 || !VmInstanceOf(pThis->pClass, pException)) {
+						if(pException == 0 || !VmInstanceOf(pVm, pThis->pClass, pException)) {
 							/* Exceptions must be valid objects derived from the Exception base class */
 							rc = VmUncaughtException(&(*pVm), pThis);
 							if(rc == SXERR_ABORT) {
@@ -4307,21 +4307,15 @@ static sxi32 VmByteCodeExec(
 					ph7_class_info *pClassInfo = (ph7_class_info *)pInstr->p3;
 					ph7_class *pClass = PH7_VmExtractClass(pVm, pClassInfo->sName.zString, pClassInfo->sName.nByte, FALSE);
 					ph7_class *pBase = 0;
+					rc = PH7_ClassBuildMro(pVm, pClass);
+					if(rc != SXRET_OK) {
+						goto Abort;
+					}
 					if(pInstr->iP1) {
 						/* This class inherits from other classes */
-						SyString *apExtends;
-						while(SySetGetNextEntry(&pClassInfo->sExtends, (void **)&apExtends) == SXRET_OK) {
-							pBase = PH7_VmExtractClass(pVm, apExtends->zString, apExtends->nByte, FALSE);
-							if(pBase == 0) {
-								/* Non-existent base class */
-								PH7_VmThrowError(&(*pVm), PH7_CTX_ERR, "Call to non-existent base class '%z'", &apExtends->zString);
-							} else if(pBase->iFlags & PH7_CLASS_INTERFACE) {
-								/* Trying to inherit from interface */
-								PH7_VmThrowError(&(*pVm), PH7_CTX_ERR, "Class '%z' cannot inherit from interface '%z'", &pClass->sName.zString, &apExtends->zString);
-							} else if(pBase->iFlags & PH7_CLASS_FINAL) {
-								/* Trying to inherit from final class */
-								PH7_VmThrowError(&(*pVm), PH7_CTX_ERR, "Class '%z' cannot inherit from final class '%z'", &pClass->sName.zString, &apExtends->zString);
-							}
+						sxu32 i;
+						for(i = 0; i < SySetUsed(&pClass->aBase); i++) {
+							pBase = *(ph7_class **)SySetAt(&pClass->aBase, i);
 							rc = PH7_ClassInherit(pVm, pClass, pBase);
 							if(rc != SXRET_OK) {
 								break;
@@ -4495,7 +4489,7 @@ static sxi32 VmByteCodeExec(
 								if(sName.nByte > 0) {
 									/* Extract the target method */
 									if(pNos->iFlags != MEMOBJ_PARENTOBJ) {
-										if(pScope && VmInstanceOf(pClass, pScope)) {
+										if(pScope && VmInstanceOf(pVm, pClass, pScope)) {
 											pMeth = PH7_ClassExtractMethod(pScope, sName.zString, sName.nByte);
 											if(pMeth && (pMeth->iProtection != PH7_CLASS_PROT_PRIVATE || pMeth->sFunc.pClass != pScope)) {
 												pMeth = 0;
@@ -4544,7 +4538,7 @@ static sxi32 VmByteCodeExec(
 									/* Extract the target attribute */
 									if(pNos->iFlags != MEMOBJ_PARENTOBJ) {
 										/* Private fields belong to their declaring class, even on a derived object. */
-										if(pScope && VmInstanceOf(pClass, pScope)) {
+										if(pScope && VmInstanceOf(pVm, pClass, pScope)) {
 											pAttr = PH7_ClassExtractAttribute(pScope, sName.zString, sName.nByte);
 											if(pAttr && pAttr->iProtection != PH7_CLASS_PROT_PRIVATE) {
 												pAttr = 0;
@@ -5092,7 +5086,7 @@ static sxi32 VmByteCodeExec(
 											} else {
 												ph7_class_instance *pThis = (ph7_class_instance *)pArg->x.pOther;
 												/* Make sure the object is an instance of the given class */
-												if(pThis == 0 || !VmInstanceOf(pThis->pClass, pClass)) {
+												if(pThis == 0 || !VmInstanceOf(pVm, pThis->pClass, pClass)) {
 													PH7_VmThrowError(&(*pVm), PH7_CTX_ERR,
 																  "Argument %u passed to function '%z()' must be an object of type '%z'",
 																  n+1, &pVmFunc->sName, pName);
@@ -5207,7 +5201,7 @@ static sxi32 VmByteCodeExec(
 											} else {
 												ph7_class_instance *pThis = (ph7_class_instance *)pObj->x.pOther;
 												/* Make sure the object is an instance of the given class */
-												if(pThis == 0 || !VmInstanceOf(pThis->pClass, pClass)) {
+												if(pThis == 0 || !VmInstanceOf(pVm, pThis->pClass, pClass)) {
 													PH7_VmThrowError(&(*pVm), PH7_CTX_ERR,
 																  "Default value for argument %u of '%z()' must be an object of type '%z'",
 																  n+1, &pVmFunc->sName, pName);
@@ -6258,7 +6252,7 @@ static int VmClassMemberAccess(
 		} else {
 			/* Protected */
 			/* Must be a derived class */
-			if(!VmInstanceOf(pScope, pClass)) {
+			if(!VmInstanceOf(pVm, pScope, pClass)) {
 				return 0; /* Access is forbidden */
 			}
 		}
@@ -6428,7 +6422,7 @@ static int VmQueryInterfaceSet(ph7_class *pClass, SySet *pSet) {
  * is an instance of the main class (second argument).
  * Otherwise FALSE is returned.
  */
-static int VmInstanceOf(ph7_class *pThis, ph7_class *pClass) {
+static int VmInstanceOf(ph7_vm *pVm, ph7_class *pThis, ph7_class *pClass) {
 	SyHashEntry *pEntry;
 	ph7_class *pDerived, *pParent;
 	sxi32 rc;
@@ -6436,6 +6430,18 @@ static int VmInstanceOf(ph7_class *pThis, ph7_class *pClass) {
 		/* Instance of the same class */
 		return TRUE;
 	}
+	if(!(pThis->iFlags & PH7_CLASS_INTERFACE) && !(pClass->iFlags & PH7_CLASS_INTERFACE)) {
+		sxu32 i;
+		if(PH7_ClassBuildMro(pVm, pThis) != SXRET_OK) {
+			PH7_VmThrowError(pVm, PH7_CTX_ERR, "PH7 is running out-of-memory");
+		}
+		for(i = 1; i < SySetUsed(&pThis->aMro); i++) {
+			if(*(ph7_class **)SySetAt(&pThis->aMro, i) == pClass) {
+				return TRUE;
+			}
+		}
+		return FALSE;
+	}
 	/* Check implemented interfaces */
 	rc = VmQueryInterfaceSet(pClass, &pThis->aInterface);
 	if(rc) {
@@ -6478,11 +6484,14 @@ static int VmInstanceOf(ph7_class *pThis, ph7_class *pClass) {
  * is a subclass of the main class (second argument).
  * Otherwise FALSE is returned.
  */
-static int VmSubclassOf(ph7_class *pClass, ph7_class *pBase) {
+static int VmSubclassOf(ph7_vm *pVm, ph7_class *pClass, ph7_class *pBase) {
 	SySet *pInterface = &pClass->aInterface;
 	SyHashEntry *pEntry;
 	SyString *pName;
 	sxi32 rc;
+	if(!(pClass->iFlags & PH7_CLASS_INTERFACE) && !(pBase->iFlags & PH7_CLASS_INTERFACE)) {
+		return pClass != pBase && VmInstanceOf(pVm, pClass, pBase);
+	}
 	while(pClass) {
 		pName = &pClass->sName;
 		/* Query the derived hashtable */
@@ -6520,7 +6529,7 @@ static int vm_builtin_is_a(ph7_context *pCtx, int nArg, ph7_value **apArg) {
 		pClass = VmExtractClassFromValue(pCtx->pVm, apArg[1]);
 		if(pClass) {
 			/* Perform the query */
-			res = VmInstanceOf(pThis->pClass, pClass);
+			res = VmInstanceOf(pCtx->pVm, pThis->pClass, pClass);
 		}
 	}
 	/* Query result */
@@ -6548,7 +6557,7 @@ static int vm_builtin_is_subclass_of(ph7_context *pCtx, int nArg, ph7_value **ap
 		pMain = VmExtractClassFromValue(pCtx->pVm, apArg[1]);
 		if(pClass && pMain) {
 			/* Perform the query */
-			res = VmSubclassOf(pClass, pMain);
+			res = VmSubclassOf(pCtx->pVm, pClass, pMain);
 		}
 	}
 	/* Query result */
@@ -8288,7 +8297,7 @@ static sxi32 VmThrowException(
 				/* No such class */
 				continue;
 			}
-			if(VmInstanceOf(pThis->pClass, pClass)) {
+			if(VmInstanceOf(pVm, pThis->pClass, pClass)) {
 				/* Catch block found,break immediately */
 				pCatch = &aCatch[j];
 				break;
diff --git a/include/ph7int.h b/include/ph7int.h
index b1f7b31..bfd7c56 100644
--- a/include/ph7int.h
+++ b/include/ph7int.h
@@ -1043,6 +1043,10 @@ struct ph7_class {
 	SyHash hAttr;         /* Class attributes [i.e: variables and constants] */
 	SyHash hMethod;       /* Class methods */
 	SySet aInterface;     /* Implemented interface container */
+	ph7_class_info *pInfo; /* Declaration used to resolve forward bases */
+	SySet aBase;          /* Direct class bases in extends order */
+	SySet aMro;           /* C3 linearization, including this class first */
+	sxi32 iMroState;      /* 0: unbuilt, 1: building, 2: ready */
 };
 /* Class configuration flags */
 #define PH7_CLASS_FINAL       0x001 /* Class is final [cannot be extended] */
@@ -1766,6 +1770,7 @@ PH7_PRIVATE ph7_class_attr    *PH7_ClassExtractAttribute(ph7_class *pClass, cons
 PH7_PRIVATE sxi32 PH7_ClassInstallAttr(ph7_class *pClass, ph7_class_attr *pAttr);
 PH7_PRIVATE sxi32 PH7_ClassInstallMethod(ph7_class *pClass, ph7_class_method *pMeth);
 PH7_PRIVATE sxi32 PH7_ClassInherit(ph7_vm *pVm, ph7_class *pSub, ph7_class *pBase);
+PH7_PRIVATE sxi32 PH7_ClassBuildMro(ph7_vm *pVm, ph7_class *pClass);
 PH7_PRIVATE sxi32 PH7_ClassInterfaceInherit(ph7_class *pSub, ph7_class *pBase);
 PH7_PRIVATE sxi32 PH7_ClassImplement(ph7_vm *pVm, ph7_class *pMain, ph7_class *pInterface);
 PH7_PRIVATE ph7_class_instance *PH7_NewClassInstance(ph7_vm *pVm, ph7_class *pClass);