SUMMARY The serological relationships of 26 strains of rumen Bacteroides species comprising Bacteroides ruminicola, B. amylophilus and B. succinogenes were examined using precipitation, agglutination and fluorescent antibody techniques. With precipitation techniques two types of acid-soluble thermostable antigens were detected, a strain-specific antigen and an antigen demonstrating wider serological relationships. With B. amylophilus and B. succinogenes this latter antigen was species-specific. With B. ruminicokl, although eight out of 12 strains possessed the same species-specific antigen, there was evidence of two other serological groups within the species. Agglutination tests detected only the strain-specific antigens. Fluorescent antibody tests detected species-specific antigens of B. amylophilus and B. succinogenes but with B. ruminicola only the strain-specific antigens could be demonstrated. Preliminary work with B. ruminicola indicated that both kinds of antigen were polysaccharides. Eleven out of 20 freshly isolated strains of B. ruminicola reacted with the B. ruminicola antiserum. When 30 strains of eight other species of Bacteroides were tested against the species-specific antiserum of the different rumen species of Bacteroides, four strains of B. oralis and one strain of B. melanogenicus were found to have a common antigen with that of B. ruminicola, and another strain of B. oralis had a common antigen with another B. ruminicola strain. No other cross-reactions were observed.