The Eurasian Rhamnus cathartica is widespread, common, and naturalized in parts of the northeastern United States. Other nonnative buckthorn species are also present but underreported because intraspecific leaf polymorphisms, hybridization, and imprecise literature descriptions confuse identifications. A study of Rhamnus populations in Tompkins and Onondaga counties in New York and Hartford County in Connecticut provided an opportunity to revisit the literature and to propose identification aids based on infrequently reported fruit stone number and mature fruit pulp color. These characters facilitate inferences of ancestry of apparent hybrids involving R. cathartica, R. davurica, and R. utilis.