Young-of-the-year largemouth bass (Micropterus salmoides) were exposed to pH levels from 8.0 to 4.5 in two water types, 1.5 and 13.4 mg Ca/L. Exposures were conducted at 3.8 deg C for 113 d, followed by 14 d of increasing temperature to 18 deg C. Two treatments in the softer water, one each at pH 5.0 and 4.5, had Al added to attain 30 micrograms Al/l; all other treatments were at approximately 5 micrograms Al/l. The condition factor of fish in all treatment groups declined with exposure time at 3.8 deg C. Fish in the 13.4 mg Ca/L water maintained osmotic homeostasis through pH 5.0 In the 1.5 mg Ca/l water, osmotic homeostasis was lost at pH 4.5 and at pH 5.0 when Al was added. Mortalities were most prevalent when exposed in the 1.5 mg Ca/l water with added Al. The probability of survival was directly correlated with blood osmolality; no correlation was found between survival probability and condition factor. A rise in blood osmolality occurred among fish from most exposure groups when the temperature was increased to 18 deg C. When fish from these chronic treatments were challenged at pH 3.8, they had shorter survival times in the softer water and after longer preexposures.