This study examines the effect of routine gamma radiation based on cobalt 60 on the viability, mosquito larvicidal activity, and density of bacillus spores in the soil. Although 1 g of unirradiated powder of Bacillus thuringiensis israelensis (Bti) contains on average 6.2 x 10(9) spores, no spores survived radiation doses of 20.6 kGy and higher. Radiation at a dose of 20-25 kGy caused a 20-30% reduction in the effectiveness of Bti powder against mosquito larvae. In areas treated with unirradiated Bti material on average twice a year, soil contained 700,000 to 44 million spores per gram. In areas treated with irradiated Bti products, either no Bti spores or fewer than 100,000 were found per gram of soil. A radiation dose of 25 kGy fulfills the requirements of killing all spores in a Bti product and maintaining the effectiveness of the product in routine treatments. No viable spores remain in water used for household purposes or irrigation of garden areas when irradiated Bti fizzy tablets are used in water containers. Irradiation of Bti products fulfills the requirements of drinking water regulations and thus allows these products to be used widely.