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Fathead minnows (Pimephales promelas Rafinesque) were continuously exposed to reduced pH levels of 4.5, 5.2, 5.9, 6.6 and 7.5 (control) during a 13-month, one-generation test. Survival was not affected, even at the lowest pH tested. Fish behavior was abnormal, and fish were deformed at pH 4.5 and 5.2. Egg production and egg hatchability were reduced at pH 5.9 and lower, and all eggs were abnormal. A pH of 6.6 was marginal for vital life functions, but safe for continuous exposure. Free carbon dioxide, liberated by the addition of sulfuric acid to reduce the pH, may have had an unknown effect. The fish did not become accliminated to low pH levels.