A fundamental question in biology concerns the morphology of spiroplasmas: How does a wall-less microorganism maintain its characteristic morphology as a helical filament? An answer to this question began to form when it was discovered that spiroplasmas treated with any of a number of detergents (sodium deoxycholate, Triton X-100, Nonidet P-40) release their cytoplasmic contents. If this procedure is performed on a formvar-coated electron microscope grid and the resultant preparation negatively stained and observed by transmission electron microscopy, numerous striated microfibrils can be seen where spiroplasmas once were. The fibrils are of varying lengths, 4 nm in width, and show a striation repeat at 9 nm along their length. It is not possible to discern from the pattern of the released fibrils just how they are organized within the intact spiroplasma; nor is it yet possible to identify a fibrillar substructure in thin sections or in freeze-fractured organisms. Townsend and his colleagues at the John Innes Institute in Norwich, UK, purified fibrils by density gradient centrifugation. SDS-PAGE showed the fibrils to consist of a 55,000-dalton protein recognizable in the four serogroups tested by protein blotting with an antiserum made against the PAGE-separated protein. The presence of fibrils is a feature common to all spiroplasma, regardless of whether they are helical or nonhelical, as in the Ixodes tick-derived spiroplasma or Townsend's ASP-1 strain of Spiroplasma citri. We have employed gentle demembranation treatments that preserve filamentous substructure in an effort to elucidate the organization of the fibrils within the spiroplasma cell.