The toxicity of Diazinon to sheepshead minnows is described. Although the number of eggs spawned by continuously exposed fish was significantly reduced (alpha =0.05) in all concentrations, no concentration tested significantly affected parental survival or fertility of eggs, nor were survival and growth of progeny affected in a subsequent 28-day test. Fish exposed to 0.47 micrograms/liter spawned 22.6 eggs/female-day, or 69% of control production (32.8 eggs/female-day). The number of eggs spawned by fish exposed to 6.5, 3.5, 1.8, and 0.98 micrograms/liter Diazinon was similar (14.9-17.9 eggs/female-day), averaging 45-55% of production by control fish. Egg production by fish that had been previously exposed to 3.5 micrograms/liter did not change significantly following depuration for 23 to 31 days. AChE activity varied inversely with exposure concentration, fish in the highest concentration (6.5 micrograms/liter) averaging 71% inhibition. The concentration of Diazinon measured in adult fish exposed to 1.8, 3.5, and 6.5 micrograms/liter averaged 169 times the concentration measured in the water. Diazinon was not consistently detected (<0.05 mg/kg) in fish exposed to 0.98 and 0.47 micrograms/liter. The maximum acceptable toxicant concentration (MATC) for sheepshead minnows continuously exposed to Diazinon, based on reduced fecundity, is <0.47 micrograms/liter; the application factor (MATC divided by 96-hour median lethal concentration) is <0.0003.