Water-soluble protein extracts were obtained from representative strains of eleven species of Legionella and examined by polyacrylamide slab gel electrophoresis (PAGE). The eleven species included: L. pneumophila (serogroups 1-9), L. micdadei, L. longbeachae (serogroups 1 and 2), L. gormanii, L. dumoffii, L. bozemanii, L. jordanis, L. oakridgensis, L. wadsworthii, L. feeleii and L. maceachernii. Species identification was verified by gas-liquid chromatography of the non-hydroxy, monohydroxy and dihydroxy fatty acids. Strains within each species shared common patterns of protein banding. Each species had a distinctive protein banding pattern. Results indicate that PAGE of water soluble protein extracts is a useful technique for identification of Legionella isolates.