The survival times of hatchery reared year‐old brown trout (Salmo trutta) were tested in solutions containing various ions at pHs between 3.5 and 4.0. The effect of calcium in prolonging the survival time compared with that in deionized water at all pHs tested, was more marked than that of sodium which was only effective at pH 4.0. Potassium and magnesium had no effect individually, but aluminium prolonged the survival time in sodium solutions. Fish from a naturally acid river (Tovdal‐Norway) survived significantly longer than hatchery reared fish, indicating the importance of the previous history of the fish to their sensitivity to low