The American lobster, Homarus americanus, lives in the cold waters of the Northeastern United States and Canada. Repeated mass mortalities of H. americanus were reported in the late summer and early fall of 1998 through present (2003) in the Long Island Sound. Increased water temperatures, exceeding the thermal limit, and water contamination were recorded in association with the mass mortalities. Affected lobsters presented with lethargy, poor shelf-life, and an orange abdominal coloration. Hemolymph samples were collected and necropsies were perfromed. Abnormalities noted on clinical chemistry included decreased glucose, protein, calcium, and iron hemolymphconcentrations. Histological changes, including multifocal granulomas and calcium carbonate crystals, were noted in various tissues. The histological changes are likely a sequela to an acid base disturbance. The histiological features defined are likely indicators of water quality stress.