Fifty clinical and environmental isolates of Legionella pneumophila were typed serologically and by DNA fingerprinting using arbitrarily primed polymerase chain reaction (AP-PCR). Furthermore, variability in and around ribosomal operons was assessed by conventional ribotyping and PCR-mediated amplification of the spacer region separating the 16S and 23S genes. It appears that serotyping suffers from low resolution capabilities, and ribotyping and spacer PCR display intermediate resolving capabilities, whereas AP-PCR is more discriminating. Results from AP-PCR and both forms of ribotyping analysis correlate with epidemiological and environmental data. It is suggested that AP-PCR typing may be the method of choice for rapidly determining clonality among L. pneumophila isolates.