β-Exotoxin (thuringiensin) was found in high titers in centrifugation supernatants and acetone/lactose powders produced from centrifugation pellets of strains Guat 1 and HD 2 of Bacillus thuringiensis (Berliner). Diets containing powders of either strain were toxic, diets containing Guat 1 supernatant were not toxic, diets containing HD 2 supernatant were slightly toxic, and diets containing powders or supernatants from uninoculated culturing medium spiked with β-exotoxin were not toxic. Most mortality occurred within 3 d when flies fed on powders but not until 6–7 d when flies fed on HD 2 supernatant. These results indicated that the primary toxic principals of the powders were endotoxins/spores and that β-exotoxin alone was not toxic to adult flies at the concentrations found in the supernatants or powders.