Three hundred and twenty-two strains of Bacteroides fragilis isolated from infected patients at three different hospitals were tested against 20 type specific B. fragilis antisera using the tube agglutination technique. Of these strains 41.3% were assigned to a single O-serotype, a further 20.5% were agglutinated by several antisera and could not be classified and the remainder showed no reactions. Three different serotypes were prevalent in the three hospitals and minor geographical variation was observed. No correlation was found between serotypes and the origin of infection, but those from the blood were the most readily typable strains. No correlation was found between serotypes and biotypes of B. fragilis.