Sensitivity, expressed as the 96-h LC50 derived from acute lethality tests, was compared for four ages (day-of-hatch, 7-d, 14-d, and 28-d) of three atherinid fishes: Leuresthes tenuis (California grunion), Menidia menidia (Atlantic silverside), and Menidia peninsulae (tidewater silverside). Responses of each age-species combination exposed to the organophosphate insecticide, chlorpyrifos, and the carbamate herbicide, thiobencarb, were compared in both static and flowing seawater toxicity tests. Chlorpyrifos was highly toxic to all atherinids (96-h LC50's ranged from 0.4 to 6.7 micrograms/L); toxicity of thiobencarb was approximately two orders of magnitude lower (LC50 values from 199 to 1405 micrograms/L). Acute lethal responses to each pesticide were similar among the three species. Sensitivity was generally highest for 7-d and 14-d age groups, and flowing-water tests were more sensitive measures of toxicity than static tests, especially for chlorpyrifos.