Tefluthrin, a highly lipophilic synthetic pyrethroid insecticide, was tested on adult Tubifex tubifex, and larval Chironomus riparius, under various substrate and temperature conditions. Acute 48-hour water-only tests at 18\sp∘C, 23\sp∘C, and 28\sp∘C showed that the emulsifiable concentrate formulation of tefluthrin is highly toxic to both tubificids (EC50 = 104 μg/L, 100 μg/L, 440 μg/L, respectively) and chironomids (EC50 = 25.6 μg/L, 24.2 μg/L, 11.2 μg/L, respectively). The toxicity of tefluthrin to chironomids in chronic 23\sp∘C 10-day sand tests (0.075% organic carbon (o.c.)) was 28.9 μg/kg, which was similar to the acute EC50 in water at 23\sp∘C. As the organic carbon content of the substrate increased, the toxicity of tefluthrin decreased. Ten-day LC50s of chironomids in sediment (10.8% o.c.) at the three temperatures were 6.37 mg/kg, 5.01 mg/kg, and 9.28 mg/kg, respectively. The 23\sp∘C chronic chironomid LC50 in a sediment/sand mix (5.1% o.c.) was similar to that for sediment at 3.56 mg/kg. Sublethal endpoints, such as larval growth and development, were affected at concentrations similar to the chronic LC50s. Adult tubificid mortality, as determined by 28-day exposures to tefluthrin, was approximately 100 times less than in acute water-only tests. Reproductive success appeared to be a more sensitive measure than adult mortality.