A strain of Legionella pneumophila (designated Concord 3) was isolated from a postmortem lung specimen of a patient with pneumonia. Direct fluorescent-antibody and immunoelectrophoresis tests, using absorbed and unabsorbed reagents prepared to this and other strains of legionellae, indicated that the Concord 3 strain represents a new serogroup of L. pneumophila, serogroup 8.