Abstract Two groups of lobsters were maintained for 31 days at temperatures environmentally realistic for Long Island Sound to investigate the effects of prolonged thermal stress on the physiology of lobsters. One group was held at 16°C, representative of late spring (controls), and the other group at 23°C, representative of late summer/early fall (treatments). In vivo hemolymph pH and samples for serum chemistry analysis were taken before and after temperature exposure. Hemolymph samples were taken before, during and after temperature exposure to investigate effects on hemocyte phagocytic activity assay and total hemocyte counts. Treatment lobsters developed a significant pH acidosis. Other serum index changes included marked hyperchloremia and hyperproteinemia. Phagocytic activity of hemocytes was significantly depressed (~60%) in treatment lobsters after 14 days and remained so until the end of the experiment. Similarly, total hemocyte counts increased strongly in the thermal stress group after 14 days, and remained so until the end of the experiment. Results suggest that late summer temperatures in the bottom waters of Long Island Sound may have profound deleterious effects on the physiology of lobsters. Recent changes in water temperature regimes in the bottom waters of Long Island Sound suggest that it may in the long term become inhospitable for lobster survival.