Jaguar and puma faeces (N 030) were collected in an area of 75 km 2 in the tropical forest of Tamasopo in San Luis Potosi. A dot-blot assay was developed with DNA from scats in order to differentiate between jaguar and puma. This assay employed a probe recognising cytochrome b of puma. A total of 14 prey species were found, of which 7 were used by both felids. Jaguar and puma diets in this study showed an overlap of 93.38% where collared peccary was the main prey consumed by both felids (jaguar 39.64% and puma 45.76%). White-tailed deer was also present in the diets of both felids, with a similar biomass consumed (jaguar 7.64% and puma 5.88%). Brocket deer and domestic goat appeared only in the puma's diet. With the exception of some remains from a lizard in the jaguar's diet, the rest of the prey species were comprised of medium-sized prey (1Á15 kg), with the common opossum and raccoon found for both felids. A method is presented to estimate the number of individuals consumed based on energy requirements. The overlap suggests an important competition between these species.