Flow-through acute and early-life-stage (ELS) toxicity tests were conducted with topsmelt (Atherinops affinis), a Pacific coast saltwater fish, and fenvalerate, a synthetic pyrethroid insecticide. The 96-h LC50 for juvenile fish was 0.66 micrograms/L. In the 30-d ELS test with laboratory-spawned embryos, average measured fenvalerate concentrations were nondetectable (<0.075 micrograms/L) in two control treatments, 0.14, 0.34, 0.82, 1.5, and 3.2 micrograms/L. Survival of embryos to hatching ranged from 94 to 100%, with no statistically significant differences among treatments. No fry survived exposure to fenvalerate concentrations > or = 0.82 micrograms/L; overall survival in lower concentrations and control treatments ranged from 86 to 97%. There were no consistent concentration-dependent differences in weight between fish in the carrier-control treatment and fish exposed to fenvalerate. Mean wet weights of surviving fish ranged from 16.9 mg in 0.34 micrograms/L to 20.3 mg in 0.14 micrograms/L. The average bioconcentration factor for fish exposed to 0.14 and 0.34 micrograms fenvalerate per liter was 315.