Predators that spend a large proportion of their food collecting budget on the catching and killing of prey after the prey have once been discovered are shown to specialize on prey of a certain size. Such specialist predators frequently coexist in hunting sets with other predators of similar morphology and hunting abilities. The coexistence is probably the result of size difference between the predator species, which leads to each predator species in a hunting-set taking a different set of prey species. On the basis of their differing sizes, weasels seem to comprise one such hunting set, but all weasels eat mostly mouse-sized food. It is demonstrated that weasels differ in their degree of specialization on mouse-sized food, larger weasels taking a greater size variety. This difference is possibly a coexistence-stabilizing force. Theoretically, coexistence might occur between two competing species if the larger, poorer competitor preyed on the smaller, better one. Such a situation may be present within the weasel hunting set. In general, size differences provide a potent means of permitting similar species of carnivore to coexist.