Order of Gram-negative micro-organisms with slender spiral bodies (6-500 pm long) by the flexion of which they move, being non- flagellate. Free-living, saprophytic, and parasitic forms occur. Demonstrable often only by dark-ground illumination, or by Giemsa stain, taking aniline dyes poorly if at all. Comprises the families Spirochaetaceae and Treponemataceae; only the latter is of medical importance.