Food, drug and cosmetic dye additives fed to adultMusca domesticaL. resulted in the photodynamic killing of flies subsequently exposed to natural or artificial light. Dye-fed flies remaining in darkness were unaffected. Nine of 14 dyes tested at 0.25% and 1.0% wt/wt and wt/v produced up to 100% mortality in the light-exposed animals. Light exposed dye-fed flies provided with a dry diet showed slower mortality rates than those animals receiving a liquid diet.