The aim of this study was to test genotoxicity aspects of the safety of two strains of Bacillus thuringiensis. The strains were of serotype H-1, producing thuringiensin, toxic to flies, and serotype H-14, producing endotoxin, toxic to mosquitoes, but not thuringiensin. Four preparations were tested: Tenfold concentrated cell-free culture media of serotypes H-1 and H-14, prepurified thuringiensin, and purified endotoxin. No increases in revertant colony numbers of the tester strains of Salmonella typhimurium TA98 or TA100 were observed at the dose levels used with or without metabolic activation. Infrequent slight increases in revertant colony numbers in strains TA98 and TA1538 at a very high test dose of 50 mg/plate, both in the presence and absence of thuringiensin, were probably caused by histidine present in the growth medium of B. thuringiensis. Furthermore, the effects were at most slightly dose related and not reproducible, and therefore the preparations can be considered nonmutagenic.