Abstract: The developing eggs and larvae of the Atlantic salmon(Salmo salar)were exposed to different water qualities. The contents of major ions in the eggs and larvae, mortality and growth rate were determined. Exposed to 320μg/L inorganic aluminium, mortality of the larvae increased at pH 6.0. Losses in sodium and chloride in eggs and larvae occured only at pH levels where cationic forms of Al dominate; losses in eggs occurred only in the later stage of development when active uptake of ions started. No effects were found at pH 6.5, where the preciptating Al(OH)3 dominate. Effects on K, Ca and Mg were less, prominent. Addition of seawater did not mitigate the toxic effect of aluminium. Discussion was made on the way in which aluminium influences the ion exchange in fish.