Rotenone, dihydrorotenone, rotenone hydrochloride, rotenone-bentonite (1:1), rotenone-lampblack (1:1), ground derris root, and powdered derris extract were each made into a paste with water and painted heavily on weighed glass slides. After the deposits had dried, the slides were weighed again. One group of slides was exposed to direct sunlight for ten days during April and May, a second group to sunlight for twenty days, a third group to arc-light for 240 hours, a fourth group of the three pure compounds only, to arc-light for 480 hours, and a fifth group was kept in the dark at room temperatures. The deposits on these slides were tested against culicine mosquito larvae and goldfish to determine the percentage loss of toxicity resulting from exposure to light. In all cases the exposed deposits were decidedly less toxic than the unexposed and, with one exception, the loss of toxicity increased with increasing exposure. Rotenone, rotenone-bentonite, derris root, derris extract, and rotenone hydrochloride lost more than half of their toxicity during ten days' exposure to sunlight. Their toxicity was practically destroyed by the exposure to arc-light. Since dihydrorotenone lost only one-fourth to one-third of its toxicity during the first ten days' exposure to sunlight, it was distinctly more resistant to detoxication during this period than rotenone. However, at the end of twenty days in sunlight and after exposure to arc-light it had lost toxicity to about the same extent as rotenone. Lampblack markedly reduced the loss of toxicity of rotenone during the first ten days' exposure to sunlight, and prevented further loss during the last ten days. It also gave some protection to rotenone under arc-light. Neither bentonite nor the substances occurring with rotenone in powdered derris root and derris extract protected rotenone from loss of toxicity. The unexposed powdered derris extract containing about twenty-five per cent rotenone was as toxic to mosquito larvae as pure rotenone. It was shown that the photochemical decomposition of dry rotenone, which results in loss of its toxicity to insects and fish, does not take place in the absence of oxygen.