We studied the competitive abilities of 2 strains of house flies, Musca domestica L., A14/WHO, a strain heterozygous for resistance gene(s), and bwb, a susceptible strain, at 4 densities 450, 900, 1,800, and 3,600 eggs per 100 g of medium in pure and mixed cultures in the absence and presence of malathion. Both genotypes exhibited lower survival, lower mean adult weight, and longer developmental time as the density increased in an environment Free of insecticide. There was a relative advantage of A14/WHO strain when mixed with bwb at densities >450/100 g. At high densities, bwb in mixed culture was at a disadvantage relative to its performance in pure culture. In an environment viith insecticide only the males of the A14/WHO strain survived. During these experiments, we observed a parabolic relationship between the egg density and the number of emerging adults in control experiments without malathion treatment. When cultures were treated with malathion, however, this relationship became linear and number of emerging adults increased with egg density. Malathion treatment reduced the intensity of larval competition at high densities by killing part of the population, thereby flies.