Diversity in the ruminal bacterial species Selenomonas ruminantium has been investigated by DNA fingerprinting, DNA-DNA hybridization, plasmid analysis, bacteriophage sensitivity, and monoclonal antibody-based immunoassay. Twenty different isolates from the sheep rumen were initially classified morphologically and by carbon source utilization. DNA fingerprint analyses and quantitative genomic DNA hybridizations showed that limited grouping of these isolates was possible, with the largest group comprising four isolates, and two other groups comprising two isolates each. The remaining isolates were unique. Plasmids in four different size classes, 2.5, 3.7, 6.5 and 12.0 kpb, were identified, but these did not appear in all isolates. There was no apparent relationship between DNA fingerprint pattern and plasmid content. Only three isolates were sensitive to the S. ruminantium-specific temperate bacteriophage S-1. These data indicate that substantial genetic diversity exists within the ruminal species S. ruminantium, but that at least one strain may represent up to 20% of isolates.