Abstract 1. 1. Groups of six hermit crabs with shells, Pagurus longicarpus Say, were exposed to one of 30 different temperature-salinity conditions. 2. 2. At low temperatures of 2 and 6°C, the osmolarity of the blood of the hermit crabs was isosmotic to the medium, while at higher temperatures of 10, 14 and 18 C, the osmolarity of the blood ranged from 200–450 mOsm/1 above that of the medium at most salinities. 3. 3. We hypothesize that at the higher temperatures, the hermit crabs sustained metabolic rates high enough to support hyperosmotic regulation while at the lower temperatures they did not. 4. 4. Low temperature-low salinity conditions were most stressful as evidenced by high mortality, swelling and motor impairment.