We used mark–recapture and removal trapping during 37 months to examine structure of small-mammal assemblages in agricultural and pastoral areas on the western llanos of Venezuela. Among 34 sites sampled, species richness, diversity, population densities, and relative contribution to the assemblage by 10 rodent and 3 marsupial species varied by habitat or land-use category, major vegetative formation, and (within agricultural systems) crop species. Most habitat types, especially relatively uniform areas of mechanized agriculture, were numerically dominated by 2 rodents, Sigmodon alstoni and Zygodontomys brevicauda. Subsistence agriculture plots were more variable and had the highest species richness and diversity. Peridomestic habitats were dominated by Rattus rattus. In contrast to findings in agroecosystems in the United States and Argentina, relative densities were not lower in crop fields than in adjacent borders nor were there differences in the structure of the rodent assemblages. Captures of Heteromys anomalus, Oecomys speciosus, and Oecomys trinitatus document range extensions for these species on the western llanos.