Jaguars (Panthera onca) and pumas (Puma concolor) are sympatric over much of their geographic range in Mexico and South and Central America. We investigated diets of these felids in and around the Chamela-Cuixmala Biosphere Reserve in western Jalisco, Mexico. Diets were determined from scat analyses and documentation of prey cadavers. Relative biomass of each prey species consumed by pumas and jaguars was estimated from analysing 65 puma and 50 jaguar scats collected from 1995 to 1998. Both jaguars and pumas fed mainly on mammals, with white-tailed deer (Odocoileus virginianus) dominating the biomass of the diet of each species (54% and 66% respectively). There was a high degree of overlap between jaguar and puma diets, but pumas had a broader food niche than jaguars, and their ability to exploit smaller prey may give them an advantage over jaguars when faced with human-induced habitat changes.