Approximately 15 species of terrestrial and freshwater mollusks are reported from Pleistocene deposits in the unglaciated Old Crow Basin of the northern Yukon Territory. Of those species, Bulimus leachi (Sheppard), a widespread Eurasian species; and Valvata piligera yukonensis (n. ssp.), closely related to an unusual Valvata now confined to Lake Baikal, U.S.S.R., are recorded from North America for the first time. Possible causes for the failure of these species of freshwater mollusks to become established in North America are considered.