Hi, For some reason it seems like a message I posted a few days ago did not get emailed to list members, but it did get archived (http://common-lisp.net/pipermail/gsll-devel/2010q2/000458.html). Here it is again: I think some additional exports are required in order to make use of the non-linear least-squares fitting outside of the gsl package. Attached is a patch that achieves this. Cheers, Russell