Hi Andrew, leastsq() can be halted by having your objective function return None. I think that doesn't work for the scalar minimizers, though I didn't try them all. Returning None from the objective function is not very fine-grained (you wouldn't know why it happened), but it seems easier to have the wrappers around the Fortran code look for a NULL return value than for a HaltError exception. It seems like a sensible, if crude, way to say "Stop now", and might be worth considering for all the minimizing functions. --Matt