Glycosylation of the flagellar core proteins (FlaBs) was detected in Treponema pallidum Nichols and in the type or reference strains of seven oral Treponema species. In several nonmotile strains of oral treponemes, the FlaBs were undetectable by both antibody and glycan staining. In contrast, a spontaneous low-motility variant of T. vincenti poundi-related strain RitzA, OMZ 305A, lacked the flagellar sheath protein (FlaA) and the two glycan-staining FlaB bands of the wild type, but antibody labeling revealed a novel FlaB band with a lower relative molecular weight. A ca. 38-kDa component of isolated endoflagella of T. vincentii OMZ 800 was identified on Western blots as FlaA by monoclonal antibody (MAb) H9-2, which specifically labels the 37-kDa FlaA protein of T. pallidum. Glycan and H9-2 labeling patterns similar to those of T. pallidum were observed in whole-cell extracts of T. medium G7201 and of 10 strains classified as T. vincentii and as two T. vincentii-related taxons. These four groups were thus identified as cultivable pathogen (T. pallidum)-related oral spirochetes as defined by labeling with MAb H9-2. No H9-2 MAb-reactive component could be detected in T. amylovorum, T. denticola, T. maltophilum, T. pectinovorum, and the three subspecies of T. socranskii.