The half-life of unpasteurized and pasteurized dried films ofBacillus thuringiensisexposed to a laboratory UV source was 30 and 45 min, respectively. Films ofB. thuringiensiscontaining peroxidase and exposed to UV were significantly more stable than those not containing peroxidase. The findings supported the assumption that sunlight inactivation of entomopathogens probably is caused by peroxide or peroxide radicals produced by the UV irradiation of amino acid (s).