As part of a larger project on biodiversity and conservation in Guyana, we documented 130 species of mammals from Iwokrama Forest. This included 7 marsupials, 4 xenarthrans, 86 bats, 5 primates, 8 carnivores, 1 perissodactyl, 4 artiodactyls, and 15 rodents. As is typical for most Neotropical sites, the 86 species of bats represent over half of the mammal diversity. Standardized collecting methods implemented in the 1997 faunal survey of Iwokrama Forest in central Guyana enabled us also to investigate species diversity and abundance resulting from the inventory of mammals. Four species of fruit-eating bats (Artibeus lituratus, A. obscurus, A. planirostris, and Carollia perspicillata) were the most abundant and accounted for 43% of the 2,097 total captures in mist nets and harp traps during 79 nights of sampling. For nonvolant mammals, terrestrial spiny rats (Proechimys spp.) represented over half (55%) of the 65 captures in primarily live box-style traps. We estimate that our inventory is approximately 70% complete with an additional 57 species of mammals expected to occur in Iwokrama Forest. More specialized field techniques are required to attain a complete inventory of mammals, and long-term monitoring should be established at several sites to study spatial and temporal variation.