Attempts at interspecific hybridization among North American Colias have been carried out by the writer in order to find out if interspecific hybridization actually occurs in nature between various North American species, and, if so, to find out how extensive it is and what are its consequences. The conditions were kept as natural as possible for this purpose. The genetics of interspecific characters were studied by rearing of progeny from successful interspecific matings and from subsequent crosses. Since the physiology and ecology of this genus are still insufficiently known, data on developmental rates, hibernation, food plants, and seasonal isolation were also obtained during the above experiments to clarify the relation between the species of the genus and also to compare the characters of hybrids with those of the parental species. These data were published separately (Ae, in press). One presumed wild hybrid was also studied.