The mode of inheritance of resistance to parathion, fenitrothion, fenthion, chlorpyr fosmethyl, and parathion-methyl was studied in a strain of C. tarsalis. Resistance in the F(1) hybrid was partially dominant. Backcross experiments indicated that resistance is derived from more than one gene. This conclusion was confirmed by repeated backcrosses of the F(1) to the susceptible strain and selection of the backcross progeny with a discriminating dosage of parathion-methyl.