We determined the diet of Chirostoma lucius from Lake Chapala and Guaracha Reservoir in central Mexico by stomach content analysis. Diet consisted primarily of juveniles of small Chirostoma species, as well as cladocerans and copepods. Seasonal change in prey showed high fish consumption in the rainy period, whereas crustacean consumption increased in the dry period. Diet varied with size, with smaller fish selecting large numbers of small zooplankton prey, whereas the largest size classes fed principally on fish. The size for the shift to piscivory was smaller in Lake Chapala (10 cm) than Guaracha Reservoir (14 cm), and the value for the pooled data of both populations was 12 cm.