We investigated the effects of immigration of individuals carrying susceptibility alleles on the level of resistance in populations of the house fly, Musca domestica L., selected for malathion resistance. Two types of males, WHO/IN males (homozygous for susceptibility genes) and A14-WHO males (heterozygous for the second chromosome) were released into the resistant populations in 1:9, 1:1, and 9:1 ratios of immigrant/resident males. In both experiments, we observed that increasing the ratio of immigrant to resident males from 1:9 to 9:1 caused an associated increase in mortality caused by malathion applications. Release of A14-WHO males was more effective in stopping the evolution of resistance in resistant populations than was release of WHO/IN males. Fitness parameters of resistant and susceptible strains were also compared. The WHO/IN strain had low viability and fecundity. R and A14-WHO males were longer-lived compared with WHO/IN males, although they did not differ in mating success.