Discriminating doses of 5% and 10% malathion for susceptible (M1s/M1s), resistant (M1r/Mr) and heterozygotes (M1r/M1s) have been obtained on colonized susceptible and resistant strains of Anopheles stephensi. Malathion resistance in this species is suggested to be under the control of a single locus on 1 of the autosomes. The alleles on the locus are codominant.