Sheepshead minnows (Cyprinodon variegatus) were exposed to the synthetic pyrethroid insecticides AC 222, 705, fenvalerate and permethrin for 28 days in early life stage toxicity tests. AC 222, 705 was 370 times more than permethrin and 30 times more toxic than fenvalerate with survival and size of newly hatched fish being the most sensitive measure of effect. AC 222,705 reduced survival of hatched fish at 0.61 micrograms/l and average weight at 0.06 micrograms/l; no effects were detected at 0.03 micrograms/l. Concentrations of AC 222,705 that diminished weights were less than the authors limits of chemical detection, 0.15 micrograms/l. Fenvalerate reduced survival of hatched fish at 3.9 micrograms/l and both weight and length at 2.2 micrograms/l; no effects were detected at 0.56 micrograms/l. Permethrin reduced survival of hatched fish at 22 micrograms/l and no effects were detected at 10 micrograms/l. The quotient of the 96-hr LC50 divided by the no-effect concentration was 0.8 for permethrin, 9 for fenvalerate and 37 for AC 222,705. The mean bioconcentration factors, concentrations measured in whole fish divided by concentrations measured in exposure water, were 480 for permethrin and 570 for fenvalerate; AC 222,705 was not detected in fish that survived the exposure.