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Stable isotope analyses can be applied to investigate niche partitioning among species. This information can reveal potential for interspecific competition for forage resources. Our objective was to determine if nilgai (Boselaphus tragocamelus), white-tailed deer (Odocoileus virginianus), and cattle (Bos spp.) diets overlap during peak growing seasons and a non-growing season in south Texas. During autumn, spring, and winter 2012-2014, we randomly collected 20 fresh fecal samples for each species across six 2 500 ha study sites on East Foundation ranches in south Texas spanning from the Gulf Coast to western south Texas. We analyzed carbon (?13C) and nitrogen (?15N) isotope signatures of fecal samples to determine the isotope niche signature for each species in relation to other species. Preliminary results suggest (1) nilgai diets overlap with deer; (2) nilgai and cattle diets overlap very little; and (3) cattle and deer diets overlap strongly only when little grass is available. Past research on nilgai, based on microhistological analysis, suggested nilgai diets are 60-70% grasses. However, during this study, even in favorable years when grasses were available, nilgai diets were < 40% grasses. Therefore, during peak growing seasons in south Texas, when forage availability is at its highest, nilgai consume a larger percent of forbs and browse, less grass than previously suggested, and have a higher probability of competing with deer during times of low forage productivity.