Genetic differences in the responses ofTribolium confusum(duVal) andTribolium castaneum(Herbst) to antibiotics are reported. The antibiotics erythromycin, tetracycline, and spectinomycin were tested for their effect on productivity of each of three strains ofT. confusumandT. castaneum. The results of two experiments examining productivity of populations in environments containing antibiotics are presented. The first experiment examines strain and species sensitivity to low dosages (0.005% by weight) of three antibiotics. ForT. castaneum, the antibiotics significantly affect productivity in only one of the three strains examined. InT. confusum, there is a significant genotype-environment interaction; that is, the different strains react in different ways to the antibiotics. The second experiment examines the effects of different dosages (0.0625–0.25% by weight) of tetracycline on the three strains ofT. confusum, and demonstrates genetic differences among strains in sensitivity to dosage of antibiotic. Differences in antibiotic sensitivity are of general interest because antibiotics are often used in laboratory insect diets.