Mammal faunas from northern French Guiana (in northeastern Amazonia) and the Yavarí-Ucayali interfluve (in southwestern Amazonia) are the first Amazonian mammal faunas to be comprehensively described in terms of taxonomic composition and community structure. Bats are the most speciose group in each fauna, followed in order of decreasing species richness by rodents, didelphimorphians, carnivorans, xenarthrans or primates, and ungulates. A total of 180 species are known from northern French Guiana and 199 species from the Yavarí-Ucayali interfluve, but an additional 25 species are possibly missing (“pseudoabsent”) from the Yavarí-Ucayali interfluvial inventory; therefore, the increment by which species richness in northeastern Peru exceeds that of northern French Guiana is on the order of 10–25%. Complementarity (dissimilarity) between these faunas is high (79%–89%) for opossums, primates, and rodents, but complementarity is substantially lower for other groups (especially bats, carnivorans, and ungulates), suggesting taxonomic differences in geographic filtering. Most species in both faunas are nocturnal, as might be expected from the abundance of bats, but even among nonflying mammals nocturnal species outnumber diurnal species by about 2:1. Approximately equal numbers of nonflying species in both faunas are arboreal or terrestrial, whereas much smaller numbers are scansorial or semiaquatic; with one possible exception, none is fossorial. Despite such behavioral similarities, these faunas differ in trophic composition and guild membership: substantially more primary consumers are present in the Yavarí-Ucayali interfluve than in northern French Guiana, whereas numbers of secondary consumers and omnivores are similar. Higher primary productivity in western Amazonia, a geomorphologically dynamic landscape with fertile soils and phenologically diverse habitats, could explain faunal differences in both species richness and trophic structure.