A total of 26 Mobiluncus strains (17 M. curtisii and 9 M. mulieris strains) were compared serologically by double immunodiffusion and immunoblotting against antisera prepared against representative isolates of each species. All strains from the same species were strongly reactive with homologous antisera but generally weakly reactive with antisera to the heterologous Mobiluncus spp. The antisera did not react with strains of the unrelated genera Campylobacter, Succinivibrio, Wolinella, Actinomyces, Anaerobiospirillum, and Anaerovibrio.