As evidenced by immediate-transition experiments, seawater adaptation in juvenile steelhead trout was independent of photoperiod and the onset of parr–smolt transformation. Increased survival occurred as the fish grew larger, and euryhalinity was reached at 12 to 13 cm. There was no marked regression in the hypoosmoregulatory mechanism in the summer. The regression reported in earlier studies was discussed in terms of seasonal changes in lipid reserves and Na+- and K+-activated adenosine triphosphatase (ATPase) activity in gill microsomes.