Osmotic responses and seasonal movements of fishes in Izembek Lagoon, a shallow embayment of the Bering Sea, were studied. Twenty-three species were collected during the study. Seven of these were year-round residents while 4 other species occur there only in the summer. Seasonal movements may be into fresh water (Gasterosteus aculeatus, Platichthys stellatus) or offshore into deeper water (Pholis laeta, Pallasina barbata, Hexagrammos octogrammus). Four of the 7 year-round residents exhibited winter serum osmotic concentrations about 60% above summer values. The relative proportion of electrolytes decreased with increasing osmotic concentration, indicating the accumulation of additional nonelectrolyte compounds in the body fluids. Although osmotic adjustments were dramatic, either supercooling or migration was still necessary to survive winter ambient temperatures.