1. Thermoregulatory behavior was studied in the Steller sea lion and in the South Australian fur seal during their respective reproductive seasons. Data were gathered using a temperature-sampling technique. 2. The rest postures used by animals of both species were closely correlated with solar radiation (as indicated by rock temperatures). At low temperatures both species used postures that concealed the flippers, thus exposing a minimum of surface area to the air. In addition, sea lions huddled against one another at low temperatures, but fur seals did not. 3. No quantitative species differences existed in behavioral responses to low (10°C) rock temperatures. At intermediate temperatures both species progressively exposed their flippers to the air. At high temperatures (30°C and above) both species resorted to the water for cooling. Female sea lions became wet at temperatures 5°C lower than did fur seals. 4. Fur seal males without direct access to water temporarily abandoned their territories and went to sea at a mean rock temperature of 33°C (approximate solar radiation level between 0.666 and 1.073 cal./cm2/min.). 5. Mass movements of females from dry areas across the rookery to the water were temperature-dependent in both species. 6. The behavior of sea lion pups showed less of a direct correlation with temperatures than for their mothers. 7. The social consequences of thermoregulatory behavior were, 1) increased frequency and intensity of sexual investigation by males toward females, 2) an increase by 11% to 15% in aggression among males, and 3) a reduction (by one half) in copulation frequency for males that abandoned their territories to gain access to water for cooling.