Six new synthetic pyrethroids, some of which possess exceptionally high mosquito larvicidal activity, were studied for their toxicity against 4 species of game and predatory fish. The various species showed different levels of susceptibility to the various compounds. As a whole most of the pyrethroids showed a good margin of safety (derived from the ratio of LC50–90fish to LC50–90mosquito larvae) to the test fish species. It is very likely that the pyrethroids with high margins of safety can be utilized in larvicidal programs without marked acute effects on fish fauna, while others possessing low margin of safety will pose a serious hazard to fish in aquatic habitats treated with these materials.