The aim of this study was to examine the latitudinal variation in preadult competitive ability of Drosophila melanogaster. Two pairs of populations from Queensland and Tasmania, Australia, were examined. Queensland flies are genetically smaller and develop more slowly than the Tasmanian flies. Survival and body size of flies raised at different temperatures and densities were compared when larvae were challenged with a common competitor. No latitudinal variation in larval survival was detected. Body size (measured as wing length) decreased with increasing temperature and larval density. Flies from the Tasmanian populations were more sensitive to the effects of temperature and density and to the joint effect of increased temperature and density. This could explain the evolution of greater growth efficiency and larger body size at lower temperatures.