Strains of H. aphrophilus, H. paraphrophilus, and A. actinomycetemcomitans are phenotypically very similar. Ribotyping of 24 strains of H. aphrophilus, 22 strains of H. paraphrophilus, 8 strains of A. actinomycetemcomitans, and one strain each of the species Pasteurella aerogenes, H. parahaemolyticus, and the genus Capnocytophaga was studied using a non-radioactive digoxigenin labelled probe based on E. coli 16S- and 23S-ribosomal RNA. Restriction fragments were generated using restriction enzyme EcoRI. The ribotypes were analysed by a numerical approach using UPGMA clustering. Two major clusters were seen: One contained all A. actinomycetemcomitans strains, the other all H. aprophilus and all except one H. paraphrophilus strain intermingled between each other. The H. paraphrophilus strain not found in the H. aphrophilus/H. paraphrophilus cluster, the H. parahaemolyticus, P. aerogenes, and the Capnocytophaga strains clustered separately from each other and the two major clusters. The H. paraphrophilus strain with the deviating ribotype was atypical in other respects: it neither did ferment lactose nor mannose and it was isolated from a deer in contradiction to the remaining H. paraphrophilus strains, which were human isolates. This study supports the view that H. aphrophilus and H. paraphrophilus should be regarded as one species separated from A. actinomycetemcomitans.