A continuous‐flow bioassay was conducted for 10 months to determine the chronic effect of zinc on fathead minnows (Pimephales promelas Rafinesque). Fish production, as based on survival, growth, and reproduction, was investigated. Reproduction by the test fish was almost totally inhibited at zinc concentrations that had no effect on survival, growth, or maturation of these same fish. At these same concentrations there was also no effect on survival of control eggs and fry. The number of eggs produced per female in the low zinc concentration (0.18 mg/l) was only 17 percent of the eggs produced in the control (0.03 mg/l of zinc). Application factors based on these data and the 96‐hour median tolerance limit (9.2 mg/l) are discussed.