A number of butadienes and their thiophene congeners were found to be phototoxic toward the larvae of the mosquito Aedes aegypti and of the fruit fly Drosophila melanogaster. Only the first instar larvae of the fruit fly were killed by the chemical treatment in the presence of long wavelength ultraviolet light (UVA), while the mosquito larvae were all affected by a similar treatment which was performed with either UVA or natural daylight. With one exception, the mosquito larvae were all killed within 1 hr of daylight exposure in the presence of an average concentration of 0.8 ppm of the phototoxin. The fruit fly larvae were killed within 20 min of exposure to UVA when in contact with a filter paper disc containing 1.13 x 10~5 g/cm2 of the phototoxin.