When exposed to a range of pH from 7.0 to 4.0 in soft water (1 mg Ca2+/L), juvenile rock bassAmbloplites rupestris, black crappiePomoxis nigromaculatus, and largemouth bassMicropterus salmoidesshowed a capacity to osmoregulate and survive for up to 30 d at pH 4.5 and above. Juvenile yellow perchPerca flavescensmaintained osmoregulatory control through 58 d at pH 5.0. All four species lost osmoregulatory control at pH 4.0, and death of fish ensued within a few days after blood osmolality declined to about 200 mosmol/kg or less (normal values, about 300 mosmol/kg). After 58 d of exposure to pH 4.0, mean blood osmolality of yellow perch was 218 mosmol/kg, and these fish were severely emaciated and moribund. Rock bass, black crappie, and largemouth bass all died by days 29, 16, and 9, respectively, when exposed to pH 4.0. However, when exposure to pH 4.0 was halted and pH was raised to 7.0 before death, blood osmolality returned to near-initial values within 15 d. Cessation of feeding also was noted at pH 4.0. Examination of gills showed progressively increased pathology with longer exposures to lower than normal pH. Among fish exposed to low pH, gill hyperplasia was present most often, but epithelial hypertrophy, chloride-cell proliferation, chloride-cell degeneration, edema, and vacuolization of the tissues also were observed. Morphological changes that were observed in the three centrarchids at pH values above pH 4.0 suggested that gill pathology may be a more sensitive indicator of potentially lethal acid stress than blood osmolality.