Criteria for separating the closely-related species Sminthopsis leucopus and S. murina were examined, and the distributions of the two forms across southeastern Australia were plotted. The ranges of the two species do not appear to overlap. S. leucopus is restricted to within 150 km of the coast in southern Victoria and southeastern New South Wales; S. murina replaces it to the north. The ranges of the two species may come into contact in southwestern and southcentral Victoria. The habitats of the two species in Victoria were compared. S. leucopus occurs in more heavily vegetated, wetter areas (vegetational cover of under- or mid-story usually greater than 50%) than does S. murina (vegetational cover usually less than 50%). These habitat preferences help to account for the specific distributions, and are interpreted in terms of the probable evolutionary history of the two forms.