Forty-one strains of Bacteroides fragilis, 20 strains of other Bacteroides species and 14 strains of other genera were examined by the indirect immunofluorescent assay (IFA) using anticapsular serum. The sixty-one Bacteroides strains were 'O' serotyped by direct agglutination tests using absorbed antisera raised against 23 strains, each with a different 'O' antigenic determinant. All 41 B. fragilis strains tested were positive by IFA with the anticapsular serum, but apart from one strain of B. distasonis, none of the remaining 19 strains of other bacteroides, i.e. B. thetaiotaomicron, B. distasonis, B. vulgatus, B. ovatus, B. melaninogenicus group and B. ureolyticus, and none of the 14 other bacterial species examined were positive. The majority of strains of saccharolytic bacteroides tested reacted with one of the 23 'O' antisera and were designated as a specific 'O' serotype; a few Bacteroides strains had multiple agglutination reactions indicating the presence of multiple antigenic determinants. All 'O' serotypes gave positive IFA tests with their homologous 'O' antisera. Common capsular determinants and 'O' antigenic determinants appear to exist on the same strains of B. fragilis. Serological typing of B. fragilis and related species would be useful in epidemiological studies.