Niche differentiation occurred on 3 dimensions in a size-structured guild of marsupial carnivores (Marsupialia: Dasyuridae) that included the Tasmanian devil (Sarcophilus laniarius), spotted-tailed quoll (Dasyurus maculatus), and eastern quoll (D. viverrinus). Diet was partitioned on a body size–prey size axis, but substantial overlap occurred between adjacent species. Complementary niche differentiation also occurred on horizontal (vegetation type and structure) and vertical (arboreal) habitat dimensions. Eastern quolls separated completely by using grasslands more than the other 2 species. Spotted-tailed quolls were distinguished by a greater degree of arboreal activity that reflected a large proportion of arboreal prey species in their diet. A strong relationship was found between body size, arboreal activity, and morphologic adaptations for climbing between predators and prey. Coevolution in phenotype among predators and prey and consequent constraint on performance in different habitat types were the most easily measured explanations for habitat differentiation among these species, particularly between spotted-tailed quolls and devils. The “ghost of competition past” is an alternative and nonexclusive explanation that can equally explain arboreal habitat use by spotted-tailed quolls and separation between the eastern quoll and the spotted-tailed quoll. Risk of predation and prey availability also may be important factors.