The relationship between diet and biomass was examined in the Amazonian ungulates (red brocket deer, grey brocket deer, collared peccary, white-lipped peccary, and lowland tapir) of Northeastern Peru. Tropical forest ungulates have lower biomasses than savanna or grassland ungulates, because in tropical forests the majority of primary production occurs in the canopy, well out of reach from terrestrial herbivores. Within the Amazonian ungulates, species that supplement their diet with animal material, namely the peccaries, obtain a greater crude and metabolic biomass, and higher reproductive rates than the purely herbivorous species. Omnivory appears to help terrestrial herbivores inhabiting closed canopy forests overcome some effects of food limitation. © 1989 Springer-Verlag.