Spirochetes are thought to remain motile in environments (such as intercellular spaces) that immobilize extracellularly flagellated eubacteria. This attribute suggests that the viscosity of the milieu is of importance to locomotion. We sought to determine the interdependence of oral spirochete locomotion with media viscosity. Video time-lapse microscopy using darkfield optics was used. The motility of the spirochetes in media of different viscosities (various concentrations of Noble agar was measured. Treponema denticola exhibited the fastest speed (18.7 ± 4.4 μ/min) at a viscosity of 30 mPa s. The highest speeds for Treponema vincentii and Treponema socranskii were 41.9 ± 14.9 and 33.4 ± 13.2 μm/min, respectively, at 88 mPa s. These data show that optimal migration of spirochetes is viscosity-dependent. The results support the hypothesis that such viscosity-dependent locomotion could be a virulence factor that enables oral spirochetes to initiate and sustain periodontal disease.