Fathead minnows (Pimephales promelas) were exposed to Dursban during a chronic toxicity test for 200 days including a reproductive period of their life cycle. The fish concentrated Dursban approximately 1700 times. Survival of first-generation fish was adversely affected at 2.68 micrograms/liter within 60 days. A significant increase in deformities occurred at 2.68 micrograms/liter within 30 days. Growth was significantly reduced at 2.68 micrograms/liter within 30 days and at 1.21 micrograms/liter by 60 days. Maturation of the first-generation fish was reduced at all Dursban exposures and reproduction was significantly reduced at 0.63 micrograms/liter and above. Growth and estimated biomass of 30-day-old second-generation fish were significantly reduced at 0.12 micrograms/liter, the lowest concentration tested. Brain acetylcholinesterase (AChE) activity was significantly inhibited at 0.27 micrograms/liter and above. AChE inhibition ranged from near 10% in fish exposed at 0.12 micrograms/liter to 89% for those exposed at 2.68 micrograms/liter. Inhibition results are compared to other results demonstrated during the chronic study. The use of exposure units (exposure concentration X exposure duration) is discussed as a tool for determining the effects of organophosphate pesticides on the environment.