The responses of Tribolium castaneum (Herbst) and T. confusum Jacquelin du Val to 23 species of seed-borne fungi and to 1 actinomycete species, cultured on potato-sucrose-agar, were tested at 31°±2°C and 70%±3% RH. Adults of both insects fed voraciously on Alternaria and Mucor sphacrosporus , but adult T. castaneum fed well also on Hormodendrum and Nigrospora , and T. confusum on Mucor silvaticus . T. castaneum laid eggs on 16, T. confusum on 10 species of fungi, and on 1 species T. castaneum laid more eggs than T. confusum . Generally, egg laying was maximum on fungi most suitable as adult food. From 8% to 77% of T. castaneum larvae developed on 8 species of fungi, their developmental periods ranging from 21.5 days on Nigrospora to 131 days on Scopulariopsis . From 3% to 60% of T. confusum completed larval development on 7 species of fungi, with larval periods ranging from 22 days on Nigrospora to 66 days on Absidia . Larval development was complete and most rapid on wheat flour-yeast (95:5) medium, and incomplete on potato-sucrose-agar medium, when these were used as controls. The pupal periods of both insect species ranged between 4 and 7 days, and were not affected by the fungal diets of the larvae. The minimum total developmental period, from egg laying to adult emergence, was 30 days for T. castaneum , 41 days for T. confusum , when larvae of both species fed on Nigrospora . Death of T. castancum larvae occurred between 3 and 12 days when fed on Streptomyces or on fungi of 15 species; larval mortality occurred within 7 days on Trichodenna and Penicillium funiculosum . T. confusum larvae died between 5 and 56 days when fed on Strcptomyces or fungi of 15 species, larval mortality occurring within 6 clays on Aspergillus fumigatus , Penicillium terrestre , and Streptomyces .