The genetics of phosphine resistance was studied in a resistant strain of the red flour beetle, Tribolium castaneum (Herbst), developed from field collections in Queensland, Australia. A program of inbreeding and selection was maintained for 5 generations to promote homozygosity in this strain and a laboratory-susceptible strain. The strain was then 12.7 times resistant in the Food and Agriculture Organization of the United Nations test on adult insects. Data from the testing of F 1 progeny from the reciprocal crosses (R ♀ × S♂ and S♀ × R♂) indicated that resistance was autosomal and semidominant with a degree of dominance -0.47 (-0.51, -0.44). Modified chi-square analysis and contingency analysis of data from the response of test crosses of F 1 progeny to both the susceptible and resistant strains and on F 2 progeny were highly significant. Low-level resistance typical of Australian populations of T. castaneum is conferred by 1 gene.