Leptinus are facultative ectoparasites known to occur on 18 species of small mammals in North America. Leptinus americanus is restricted to the central United States, west of the Mississippi River. Its hosts and seasonality are unknown. Leptinus orientamericanus n.sp. is widespread east of the Mississippi River. It occurs on 7 hosts, most frequently on Blarina brevicauda and Scalopus aquaticus and during fall and winter months. Leptinus occidentamericanus n.sp. has a range in western North America from California to Alaska and the Yukon. It occurs on 11 hosts, most frequently on Sorex trowbridgii, and during spring and fall months. A hypothesis of evolution for all 9 species in the genus suggests a Palaeoarctic origin for the family and genus and an early Tertiary entry to North America by a Bering land bridge.-Author