Abstract
1. A study has been made of the survival and osmotic regulation of young salmonid fishes following transfer from fresh water to various dilutions of sea water 2. Survival is in the order: Salmo salar > S. gairdnerii > S. trutta and is generally better the larger the fish. 3. The survival pattern of alevins differs from that of the older stages. 4. Hypo-osmotic regulation is first seen in parr and becomes fully effective in smolts.