Amoebae are natural hosts for legionellae. The significance of this with regard to the multiplication of legionellae in the environment, the protection of legionellae from biocides, and the epidemiology of Legionnaires' Disease and Pontiac fever are discussed. A vesicle, or small amoeba, containing many legionellae is suggested as the main infective particle for man. We believe Pontiac fever can be explained as involving hypersensitivity to acanthamoebae, and comment on the lack of phages for legionellae. Legionella-like amoebal pathogen 1 (LLAP-1) was isolated, and probably belongs to a new species of Legionella. Further amoebal hosts for legionellae were found.