The roles of the fimbrial subunit and the putative basal protein antigens in the serological classification of Bacteroides nodosus have been examined by Western blot (immunoblot)-antibody binding studies of fimbriae isolated from a wide range of strains representative of different serogroups and serotypes. Fimbrial subunits were recognized by antiserum against the homologous serogroup but not generally by heterologous antisera, whereas recognition of the basal antigen was independent of serological classification. Secondary cross-reaction patterns among fimbrial subunits indicated that some serogroups may be more closely related than others. Examples include serogroups C and G and serogroups D and H. Similar analyses of isolates classified within serotypes A1 and A2, with serotype-specific antisera, showed that this subdivision is also determined by the fimbrial subunit and that significant variation does occur even at this level. These studies suggest that the various serogroups and serotypes of B. nodosus comprise a series of overlapping sets of antigenically related strains.