It has to do with the indexing type for what I see. Don't know how that works exactly. If using VisualWorks is as easy as changing the indexing type to Object at the NewClass wizard in the advanced properties. Although I don't know what that actually does, but it fix the problem. I don't know how to achieve that same thing in Pharo. But knowing that maybe someone with real knowledge can help. The problem is: class Foo sublcasing ArrayedCollection Foo>>mynew ^super new: 10. The Error ' cannot have variable sized instances' is launched in Beahavior Behavior>>basicNew: sizeRequested "Primitive. Answer an instance of this class with the number of indexable variables specified by the argument, sizeRequested. Fail if this class is not indexable or if the argument is not a positive Integer, or if there is not enough memory available. Essential. See Object documentation whatIsAPrimitive." <primitive: 71> self isVariable ifFalse: [self error: self printString, ' cannot have variable sized instances']. (sizeRequested isInteger and: [sizeRequested >= 0]) ifTrue: ["arg okay; space must be low." OutOfMemory signal. ^ self basicNew: sizeRequested "retry if user proceeds"]. self primitiveFailed I try redefining isVariable so it returns true, but then the error of OutOfMemory raises. Sorry for not being of much help, I've just started with Pharo, and I'm really used to VW (Where you don't actually know whats going on) so I still can't do much in Pharo. -- Alan Rodas Bonjour