The aquatic toxicology of the photostable synthetic pyrethroid insecticides as it affects two important groups of susceptible organisms — fish and aquatic insects — is discussed. The sensitivity of these aquatic species to the pyrethroids is dependent on several factors, including toxicokinetics, target site (nervous system), sensitivity and possible secondary mechanisms of action, as well as chemical and physical properties of the aquatic medium that influence toxicity and bioavailability. Uptake rates and routes of fenvalerate greatly affected the toxicity of fenvalerate to mosquito larvae. LD50 values were determined for cuticular and dietary exposure routes by utilizing radiolabeled fenvalerate at the respective LC50 concentrations in the two media (water and food). Fenvalerate was sixfold more toxic to mosquito larvae by the cuticular route. Technical fenvalerate was more toxic to larvae than was the emulsifiable concentrate formulation. Addition of different concentrations of humic acid to the water reduced the toxicity to the larvae. Review and analysis of relevant literature are integrated into a discussion of the principles and details of aquatic toxicology of the pyrethroids.