Although roundtail chub (Gila robusta) is generally considered a riverine species, 6 natural lakes in the upper Green River basin, Wyoming, have resident populations of this fish. In 2 of the lakes, Halfmoon and Little Halfmoon, we investigated the ecology of resident roundtail chub, including their habitat use, diet, weight—length relationships, growth, and reproduction. Lentic roundtail chub used littoral and mid-depth benthic habitats most often, with the highest catch rates in littoral habitats. Roundtail chub were rarely caught in pelagic or deep benthic habitats. Opportunistic foraging was observed in both lakes, with roundtail chub consuming terrestrial and aquatic insects, vegetation, and fish. Roundtail chub from both lakes had weight—length relationships similar to those reported for lotic populations but slower annual growth rates. Fish in spawning condition were captured from mid-June to late July when water temperatures ranged from 8.7 to 18.3 ^°C and snowmelt runoff was at its maximum. Knowledge of roundtail chub ecology in lentic systems will influence management decisions and conservation actions important to preserving this species in the Colorado River basin.