Chronic effects of ammonia on the fathead minnow Pimephales promelas were studied in the laboratory in two flow-through tests, each test lasting approximately 1 year. Fish were exposed to five test concentrations over the range 0.07–0.96 mg/L un-ionized ammonia (NH3); the mean pH of the test water was 8.0, and the mean temperature was 24.2°C. The tests started with 3- to 5-d-old larvae that were reared to sexual maturity; progeny of these fish (F1were reared until they were 60 d old. The 5% probability level was chosen to indicate significance. No effects were observed on growth or survival of parental fish at 0.44 mg/L NH3, or on egg production or viability at 0.37 mg/L, but effects on all of these were observed at 0.91 mg/L. Growth and survival of F1larvae were not affected at 0.36 mg/L NH3, which was the highest concentration at which these were tested. Egg hatching success was not affected at 0.19 mg/L NH3, but was at 0.37 mg/L. Brain lesions were common in parental fish at all stages of development at exposure concentrations of 0.21 mg/L NH3and higher, but not at 0.11 mg/L; no other histopathologic effects were observed at any of the test concentrations. The chronic-effects threshold concentration, based on survival, growth, and reproductive success, is estimated to be 0.27 mg/L NH3for the conditions of these tests. Based on histological damage, however, this concentration is estimated to be 0.15 mg/L NH3.