Summary Autopsy studies demonstrated four Leptospira icterohemorrhagiae infections among 1,121 rodents trapped from different localities in the Nile Delta of Egypt. Two of the infected rodents were Rattus norvegicus, one was Rattus rattus rattus, and one was Arvicanthis niloticus niloticus. The latter is a species of grass rat which has not before been described as a reservoir of pathogenic leptospires. A serological analysis of the serum of 1,062 randomly selected human blood specimens was made to determine the presence of leptospiral antibodies. Five sera contained antibodies against leptospires—three of these indicated previous infection with L. icterohemorrhagiae and two indicated previous infection with L. grippotyphosa. Serological analysis of the sera of 246 domestic animals demonstrated antibodies in the sera of water buffalo against L. grippotyphosa (10 animals) and L. sejroe (4 animals).