The genotypic diversity of 40 presumably epidemiologically unrelated strains of Pseudomonas aeruginosa belonging to nine different O-serotypes was analysed according to ribosomal DNA fingerprints. Ribotyping was performed with a digoxigenin-labelled DNA probe and four restriction endonucleases. Characteristic banding patterns of three to 12 bands were obtained with the different endonucleases. Among the 40 strains, eight, nine, 10 and 29 different ribotypes were differentiated with EcoRI, the combination EcoRI+HindIII, BamHI and PvuII, respectively. Poor correlations were noted between the results of serotyping and those of ribotyping. With the latter method, indices of discrimination were calculated for each enzyme from the data of the 40 unrelated strains: the values ranged from 0.678 for EcoRI to 0.979 for PvuII. Epidemiologically related samples were also tested; this enabled assessment of whether the method was able to cluster strains from a common origin with each of the enzymes tested. Ribotyping with PvuII endonuclease is proposed for screening large numbers of P. aeruginosa strains in epidemiological studies. Additional enzymes could be used to further increase the discrimination between isolates found to be indistinguishable with PvuII enzyme.