Fifty-three Nocardia strains were the subject of a restriction polymorphic ribosomal RNA analysis (ribotyping) designed to distinguish between representatives of clinically significant species and related strains. The organisms were assigned to 19 groups using a combination of EcoRV gene restriction endonuclease patterns and a digoxigenin-labelled Streptomyces violaceoruber TK21 rDNA probe. Each ribotype group contained 4 to 13 restriction fragments that ranged in size from 20.710 0.9 kb. The N. brasiliensis, N. crassostreae, N. farcinica, N. otitidiscaviarum, and N. seriolea strains showed distinct ribotype patterns. Unique banding patterns were also seen for the type strains of N. brevicatena, N. carnea, N. salmonicida, N. umformis, and N. vaccinii, and for the single representatives of "N. fusca", "N. pseudosporangifera", and "N. violaceofusca". More than one banding pattern was detected for the N. asteroides, N. flavorosea, N. nova, N. pseudobrasiliensis, and N. transvalensis strains. The results are in line with current trends in nocardial systematics thereby indicating that restriction polymorphism ribosomal RNA analyses provide valuable data for the classification and identification of novel and pathogenic nocardiae at the species level.