Here's my scenario: - a Pier kernel was previously serialized with Fuel-1.7 - a class was re-named (happens to be a subclass of PRCommand) - get a walkback when trying to de-serialize with the same Fuel-1.7 - define the missing class, then de-serialize works I was hoping that, after serializing the Pier kernel again (with both classes present, to be able to load), then on the next re-start, the old class would no longer be needed. That did not work - same problem of FLClassNotFound. To debug this, I de-serialized in a development image. To my surprise, evaluating "OldClass allInstances" found 0 instances of the OldClass. So I can't figure out how to track down why Fuel is still looking for that old class. BTW, since Pier looks for all subclasses of PRCommand, I made the NewClass a subclass of Object. That did not fix the problem. Any ideas why Fuel is looking for OldClass, yet there are no instances of OldClass in the de-serialized object graph. The stack trace is included at the end. -- Yanni ===== stack trace ===== [FLClassNotFound new signal: 'Class ' , className , ' not found at materialization'] in FLGlobalClassCluster(FLGlobalCluster)>>materializeGlobalClassFrom: SystemDictionary(Dictionary)>>at:ifAbsent: FLGlobalClassCluster(FLGlobalCluster)>>materializeGlobalClassFrom: FLGlobalClassCluster>>materializeFrom: FLGlobalClassCluster(FLPrimitiveCluster)>>materializeInstanceWith: [objects add: (self materializeInstanceWith: aMaterialization)] in FLGlobalClassCluster(FLIteratingCluster)>>materializeInstancesStepWith: SmallInteger(Integer)>>timesRepeat: FLGlobalClassCluster(FLIteratingCluster)>>materializeInstancesStepWith: FLMaterialization>>clusterInstancesStep [self clusterInstancesStep] in FLMaterialization>>instancesStep SmallInteger(Integer)>>timesRepeat: FLMaterialization>>instancesStep FLMaterialization>>run [:aStream | (FLMaterialization on: aStream) run; yourself] in FLMaterializer>>setDefaultMaterialization [self verifySignatureFrom: aStream. self verifyVersionFrom: aStream. ^ materializationFactory value: aStream] in FLMaterializer>>materializeFrom: BlockClosure>>ensure: FLMaterializer>>materializeFrom: [:aFileStream | self newDefault materializeFrom: aFileStream binary] in FLMaterializer class>>materializationFromFileNamed: [anotherBlock value: file] in StandardFileStream class(FileStream class)>>detectFile:do: BlockClosure>>ensure: StandardFileStream class(FileStream class)>>detectFile:do: StandardFileStream class(FileStream class)>>oldFileNamed:do: FLMaterializer class>>materializationFromFileNamed: UndefinedObject>>DoIt