Reproduction of rainbow trout Salmo gairdneri in acidic water was studied. Adults were exposed to waters of pH 4.5, 5.0, or 5.5 during the final 6 weeks of reproductive maturation; control acidities were pH 6.5–7.1. Progeny of acid-exposed females and control males had reduced survival through 7 d of development, hatching, and yolk-sac absorption, demonstrating that oo-genesis is sensitive to acidic conditions. Similar reductions in the survival of the progeny of acid-exposed males and control females indicate the sensitivity of spermatogenesis to low ambient pH. Among the progeny of unexposed adults, survival through 7 d of development and hatching was lower during rearing at pH 4.5, 5.0, or 5.5 than at pH 6.5–7.1. No eggs exposed to pH 4.5 survived to the eyed stage. Concentrations of plasma estradiol-17β, androgens, and 17α-hydroxy-20β-di-hydroprogesterone in acid-exposed adult rainbow trout revealed no gross physiological abnormalities. Plasma calcium concentrations in adult females were unaffected by low pH, but were decreased in adult males exposed to pH 4.5 for 42 d. Plasma sodium concentrations fell slightly in adult fish after 7 d of exposure to pH 4.5, 5.0, or 5.5 and continued to decline in fish exposed to pH 4.5. We conclude that reproduction of rainbow trout is likely to be affected by environmental acidification to pH values below 5.5, although responses may be modified by other biological and physical conditions.