Three parallel lines of Drosophila melanogaster were selected with different concentrations of the insecticide malathion over a period of 70 generations. Periodic assays found that the resistance of the adult and pre-adult stages increased at a rate proportional to the selection intensity. Ultimately all selected populations achieved similar levels of resistance. Relaxation of selection resulted in resistance drop-offs to pre-selection levels, while continued selection maintained resistance at a constant level. The results suggest that physiological resistance and behavioral response to malathion have distinct genetic bases, and that resistance of adults and larvae are also under independent genetic control.