That is a weird one. Java serialization can sometimes be tricky. I think the issue is that you have a serializable object which doesn't have a default (no-argument) constructor or is subclassing from a class with the same. See: http://forums.sun.com/thread.jspa?threadID=686452 Sean On Dec 5, 2009, at 5:52 AM, John Smith wrote: > When restoring a checkpoint, I receive a strange error: > > STARTUP ERROR: > An exception was generated upon starting up from a checkpoint. > > java.io.InvalidClassException: brain.BrainHelper; brain.BrainHelper; > no > valid constructor > > This class BrainHelper is only used as a helper class to evaluate the > individual, and is not referenced by EvolutionState, the problem, or > the > Individual. I could create a default constructor that does nothing, > but I am > curious why this class is even relevant? > > Thanks for any help.