The lesser grison (Galictis cuja), a poorly known mustelid of southern South America, has been described as a catholic feeder living close to water. Consequently, the potential exists for competition with the recently introduced American mink (Mustela vison). Nonetheless, like most ferrets the lesser grison can be a specialized mammal hunter, in which case it could benefit from introduced mammalian prey (the European hare, Lepus europaeus, and European rabbit, Oryctolagus cuniculus). The diet and trophic position of the lesser grison in Argentinean Patagonia are described in order to discuss the potential effects of introduced mammalian species on grison populations. Mammals (mainly native rodents and introduced lagomorphs) occurred in 100% of the feces and represented 95% of the biomass consumed. In one site, European hares and rabbits reached a combined 66% of biomass consumed, whereas in the other site rabbits were scarce or absent and lagomorphs represented 35% of the biomass consumed. We postulate that lesser grison populations in Patagonia have benefited from the introduced lagomorphs. On the other hand, lesser grisons frequently occupied river valleys, but did not rely on aquatic or semiaquatic prey. Hence, hunting-habitat preference segregates this mustelid from the American mink (Mustela vison), which consumes a catholic diet in which aquatic species usually predominate.