Evolution of insecticide resistance, measured by the frequency of a resistant allele and by population size, was simulated on a computer. The effects of dominance, initial gene frequency, refugia, immigration, and reproductive potential were studied singly with a deterministic model. In all cases, resistance evolved eventually, although the rate at which this occurred varied. The slowest response to selection occurred when (1) the population was diluted by immigrants, (2) population density was drastically suppressed by severe selection, and (3) susceptible individuals had a reproductive advantage over their resistant counterparts.