The four different assemblages of marsupial dasyuroid carnivores in Australia, comprising four species on the island of Tasmania and three different two-species assemblages on the mainland of Australia, present an ideal opportunity to test the theories of competitively based morphological character displacement. With males and females treated as separate morphospecies, four trophic characters that relate to killing and feeding behavior and skull length, as a measure of body size, were examined for evidence of morphological patterning, latitudinal size variation, and sexual dimorphism. The competitive basis for character displacement in intra- and interspecific variation in prey size was sought from dietary analysis at a field site. Even spacing in canine strength, but not in other characters, was found among species and sexes of quolls (Dasyurus viverrinus and D. maculatus), but not including the devil (Sarcophilus laniarius) or the thylacine (Thylacinus cynocephalus), in Tasmania. Even spacing was found in temporalis muscle size for all combinations of the four Tasmanian species. Canine breaking strength and muscle strength driving the canines can be construed as the most important trophic characters involved in prey capture, and therefore in determining the upper size of prey that can be handled. Even spacing was found in Tasmania, where there are two larger species in the guild, but not in any of the three mainland assemblages where the larger species of carnivores have been absent for many generations. The basis for the observed morphological patterning in killing behavior, evidence for even spacing in prey size, and partial character release on the mainland are consistent with the hypothesis of competitive character displacement. These size relationships result from a complex interplay of factors. Latitudinally correlated size variation has influenced an increase in body size and canine strength in D. viverrinus and an increase in body size in D. maculatus in Tasmania. This is countered by decreased canine strength in D. maculatus in Tasmania resulting from competitive character displacement. The result is character convergence. Sexual selection also influences size relationships.