Both virulent and benign isolates of the ovine pathogen Dichelobacter (Bacteroides) nodosus produce polar fimbriae which have been implicated in twitching motility. The fimbrial subunit-encoding genes from two virulent and two benign serogroup-B isolates of D. nodosus were cloned and sequenced. Analysis of the deduced amino acid (aa) sequences of these subunits indicated the presence of substitutions that appeared to correlate with the virulence phenotype. However, these aa substitutions were located in variable regions of the protein where they are unlikely to alter the functional properties of the fimbriae. The aa sequences of the serogroup-B subunits had a very high level (91-95%) of similarity, particularly at the N terminus, where the conserved region extended up to aa 61. Specific aa substitutions within the subunit of one isolate may reflect its serotypic variation from the other serogroup-B subunits studied.