Epizootic shell disease is responsible for great economic losses in the commercial lobster fishing industry in the northeastern United States. Histologic examination of the carapace from affected American lobsters showed the disease is primarily caused by bacteria invading the carapace from the surface of the shell. The carapace itself appears normal in areas that are not eroded and there seems to be an appropriate inflammatory response mounted by the lobsters' hemocytes and tissues to the erosive lesions.