Insecticide assays were used to analyze the inheritance of fenitrothion resistance (145-fold) in a strain of the sawtoothed grain beetle, Oryzaephilus surinamensis (L.), from Australia. At the LC50, the response of the RS hybrid to fenitrothion was closer to its resistant than to its susceptible parent strain; there was no significant difference between the RS and SR hybrids (P < 0.05). X2 analyses of the observed responses of the F2 progeny and the F1-backcross progenies rejected the null hypothesis that a single gene was responsible for resistance. Fenitrothion resistance appears to be controlled by two or more autosomal factors and the major gene(s) involved is incompletely dominant.