Few studies have contrasted faunal communities between flooded and unflooded tropical forests, and such attempts have largely been restricted to a few taxonomic groups.We present the first comparison of the composition and structure of an entire assemblage of mid-sized to large-bodied mammals in adjacent unflooded (terra firme) and flooded (v´arzea) forests of central-western Amazonia. We extend this comparison to 13 other terra firme and v´arzea forest sites in order to examine the fundamental dichotomy between mammal communities in these Amazonian environments. We found a consistently impoverished fauna in v´arzea environments both in terms of primates and other non-volant mammals, although primate density and biomass was substantially higher in v´arzea than in terra firme. The average Bray?Curtis mammal community dissimilarity between terra firme and v´arzea forests was 74%, whereasmean dissimilarity within v´arzea and terra firme samples was 40% and 39%, respectively. The results seem to be largely a function of high habitat heterogeneity and floristic diversity in terra firme and the physical connectivity and proximity of v´arzeas to adjacent terra firme forests. We suggest that inundated forests should be set aside as a crucial complement to Amazonian reserves dominated by terra firme forests in future biodiversity conservation planning.