This study evaluated the role of diet, specifically the relative importance of salt content versus energy content, in the response of juvenile rainbow trout to environmental acid stress in soft water ([Ca2+] = 0.057 mmol L-1, [Na+] = 0.047 mmol L-1). Diets were formulated at two energy levels (regular, 16.3 MJ kg-1, and low, 9.8 MJ kg-1) and two levels of NaCl (regular, approximately 263 mmol kg-1, and low, approximately 64 mmol kg-1), yielding four treatment combinations, each fed at a ration of 0.6% body weight d-1. A fifth group of fish was not fed during the experiment. All groups were subjected to an initial acid challenge (24 h at pH 5.0 plus 12 h at pH 4.0), followed by 28 d of exposure to pH 5.2. Following the initial acid challenge, typical ionoregulatory disturbances were seen, but most effects had attenuated or disappeared by day 20 of chronic low-pH exposure. However, after 28 d, fish fed the regular-salt diets maintained the restored ionic homeostasis, whereas those fed low-salt diets did not, regardless of the energy content of the diet. Growth and food conversion efficiency were greatest in trout fed the regular-energy/regular-salt diet, negative in fish fed the low-energy/regular-salt diet, and intermediate in trout on the other diets; starved fish lost weight. Fish maintained on the regular-energy/low-salt diet exhibited the most deleterious effects, including elevated cortisol levels and a 4.1% d-1 mortality rate. Fish fed the low-energy/low-salt diet, those fed regular-salt diets, and starved fish were not as adversely affected by the acid stress. Following a regular-energy meal, fish tended to exhibit an elevated rate of oxygen consumption, but this did not occur after a low-energy meal, regardless of its salt content. Elevated oxygen consumption may be accompanied by a loss of ions via the osmorespiratory compromise. We hypothesize that fish fed the regular-energy/low-salt diets were most deleteriously affected in an acidified environment because they were unable to replace increased branchial ion losses with dietary salts. These results indicate that it is the salt content of the food rather than the energy content that is critical in protecting against the deleterious effects of low pH.