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A predominant viewpoint on the dynamics of the Plio-Pleistocene exchange of mammalian taxa between the Americas invokes competitive displacement, i.e., the notion that North American stocks outcompeted their South American counterparts by virtue of greater resilience to extinction and higher rates of colonization and diversification. A recent statistical test (Lessa and Fariña, 1996) based on South American late Pleistocene mammalian genera failed to demonstrate any significant differences in extinction rates between South and North American stocks, but showed that body mass was the primary factor statistically associated with the probability of extinction. Here, we provide additional tests of differential extinction in relation to body mass, origin, and trophic niche, using data of North American late Pleistocene mammals, alone and combined with those of the previous study. Tests were carried out using genera …