A cloned EcoRI fragment from Legionella pneumophila, which includes 16S and 23S rRNA genes, was used to identify bacteria belonging to the genus Legionella by hybridization to a series of species specific restriction fragments. Examination of the type strains of 28 species of legionellae gave different band patterns in every case. When further isolates of these species were tested the patterns obtained were usually either identical, or very similar, to those of the respective type strains. Thirty-one coded isolates were examined and of these 29 were allocated to the correct species. The remaining strains (a non-Legionella and a L. pneumophila) could not be identified using this technique. The rRNA gene probe method should be of great value in the identification of legionellae, particularly for those species which are at present very difficult to distinguish serologically.