Larvae and adults of three strains of the yellow fever mosquito Aedes aegypti L. were tested to determine the presence or absence of a diurnal rhythm of susceptibility to the organochlorine insecticides dieldrin and DDT. Each strain showed different patterns of larval susceptibility to the toxicants. Larvae and adults of the same strains produced opposite time-dependent responses to treatment. The significance of daily patterns of changing susceptibility in toxicological studies is discussed.