The genetic compatibility of the greater fritillary butterflies was tested through hybrid crosses made between the North American genus Speyeria and the Eurasian genera Argynnis and Mesoacidalia. Numerous hybrid crosses made between A. paphia and Speyeria failed to produce any viable progeny, but a hybrid male was successfully produced between M. aglaja and S. nokomis. The genetic compatibility and divergence of Speyeria species was tested through hybrid crosses and back-crosses among the various species. All species of Speyeria appear to be inter-fertile in hybrid crosses with the exceptions of the two most divergent species, S. idalia and S. diana, that produce non-viable hybrid females and sterile hybrid males. As a consequence, it is postulated that inter-species gene flow through hybridization accidents in nature has been important in the past evolutionary history of this genus, and is partly responsible for the absence of consistent, non-overlapping diagnostic taxonomic characters among most species of the genus.