A new species of bacteria that is an etiologic agent of human pneumonia has been isolated and characterized. Clinical symptoms of infection with this organism are not readily distinguishable from those caused by Legionella pneumophila infection. The organism was isolated from respiratory tract specimens from four patients. Two cases of infection apparently originated in California and one in Georgia, and a fourth was of unknown geographic origin. The name Legionella longbeachae species nova is proposed for this organism. The type strain of L. longbeachae is Long Beach 4 (= American Type Culture Collection 33462).