Flies resistant to the insect growth regulator cyromazine were selected in the F₁ generation from a cyromazine-susceptible strain of Drosophila melanogaster (Meigen) treated with ethyl methanesulfonate. Four resistant strains were isolated by screening with cyromazine at a concentration >LC₁₀₀ of susceptibles. In each strain, resistance is conferred by a single gene mutation. Cyromazine resistance in two of the mutants (rst(1a)cyr¹ and rst(1a)cyr²) localizes to map position 17 of the X chromosome. Evidence is presented that these mutations are alleles of the gene rst(1a)cyr. Cyromazine resistance in another of the mutants (Rst(1b)Cyr) is also X-linked, and localizes to map position 49 of the X chromosome. The location of the gene conferring cyromazine resistance in the other mutant (Rst(2b)Cyr) is map position 66 of chromosome II. This is possibly an allele of a previously characterized cyromazine resistance gene, Rst(2)Cyr. Dosage–mortality analyses demonstrate a low level of cyromazine resistance is conferred in all strains.