This study compares the intraspecies genetic relatedness among strains of two established species of Acholeplasma. Radiolabelled DNA probes were prepared from three strains of Acholeplasma laidlawii and two strains of Acholeplasma axanthum, by using the nick translation method. The labelled DNA probes of these two strains were hybridized to an excess of unlabelled DNA from 12 strains of Acholeplasma laidlawii and from six strains of Acholeplasma axanthum, respectively. Nucleic acid hybridization analyses showed a wide variation among strains within each of the two established species, ranging from 48 to 100% homology. The results demonstrate that strains isolated from diverse hosts and habitats within a given species of Acholeplasma exhibit extensive genotypic variations.