Treponema denticola is an oral spirochete associated with periodontal diseases. Because bacterial motility is likely to be a potential virulence factor, we investigated the effect of viscosity and temperature on cell speed. In agreement with the work of others, translational motility was a function of the macroscopic viscosity of the medium. In addition, we found that although the speed of spirochetes was slow at 25°C (4 μm s-1), it increased quite markedly at 35°C (19 μm s-1). The results indicate that both viscosity and temperature are critical factors in T. denticola translational motility. Copyright (C) 1998 Federation of European Microbiological Societies.