The phototoxicity of 2-acetylnaphthalene, 1-acetylnaphthalene, 1-naphthalenecarboxaldehyde, 9-xanthenone, 9-thioxanthenone, 9-methylanthracene, anthracene, alpha-terthienyl, pyrene and fluoranthene was tested with larvae of Aedes aegypti, Ae. taeniorhynchus and Culex quinquefasciatus. The larvae were exposed to the chemicals in the presence of sunlight for intervals from 1 to 6 hr and the corresponding LC50 values calculated. The LC50 values determined when the 6-hr exposure was followed by 18 hr in the dark range from over 10 ppm to below 0.001 ppm. Naphthalene derivatives were the least active while alpha-terthienyl, anthracene, 9-methylanthracene, pyrene and fluoranthene were the most phototoxic.