Young-of-the-year largemouth bass were exposed to simulated overwintering conditions, 4 C for 113 days, in soft water. In some treatments pH was reduced to 5.0 and 4.5, with no added Al; in others, a monomeric Al level of 30 micrograms/L was maintained. In the neutral pH treatments the bass had greatly thickened gill respiratory epithelium and large numbers of chloride cells (compared with bass kept at 20 C in a prior experiment), and blood osmolalities declined and stabilized at about 260 mosmol/kg. In the acid treatments the bass had thickened respiratory lamellae and chloride cell numbers similar to those found in the neutral pH treatments. In very soft water, mean blood osmolalities declined to 242 and 219 mosmol/kg at pH 5.0 and 4.5, respectively, and did not stabilize. In the acid + Al treatments, respiratory lamellae were frequently obliterated by hyperplasia of the inter-lamellar epithelium, suggesting markedly compromised respiratory ability. In addition, chloride cell numbers were usually depressed, and those cells that were present were often vacuolated or covered by layers of pavement epithelium; blood osmolalities declined to the lethal and near-lethal level of about 200 mosmol/kg, and heavy mortalities occurred. The authors suggest that overwintering, with only moderate elevation of acid and Al levels, may represent an important critical period for survival to young-of-the-year fish in northern climates, principally by adversely affecting electrolyte balance. The gills of juvenile largemouth bass undergo marked structural changes with prolonged exposure to cold water, possibly to retard electrolyte losses.