The anaerobic spiral-shaped bacterium Anaerobiospirillum succiniciproducens was isolated from the blood of an AIDS patient for the first time in Europe. Electron-microscopical methods, especially negative staining, allowed rapid morphological classification and differentiation from Campylobacter species. While A. succiniciproducens revealed lophotriche flagellation all the investigated Campylobacter species showed monotriche flagellation. The cell diameter of A. succiniciproducens was at least double that of the investigated Campylobacter species. Other ultrastructural features, such as a ring-like structure underneath the flagellar area and fibrils arranged parallel along the axis, were also specific to A. succiniciproducens.