The purpose of this study was to elucidate the distribution of spirochetes (SP), on the basis of ultrastructural criteria in subgingival plaque from patients with advanced marginal periodontitis. Samples were obtained by a paperpoint technique from 18 pockets 6-10 mm in depth from 12 individuals. On grids with negatively stained material, more than 50 SP were photographed from each sample. The SP were primarily grouped according to the number of endoflagella from each cell-end, with the exception of SP with more than eight endoflagella, which were pooled in one group. The diameter, the length, the wavelength and the amplitude of the SP were measured and related to the number of the endoflagella. The 95 per cent and 99 per cent confidence intervals were estimated. Owing to the number of endoflagella, the distribution of SP varied in the separate samples and no relationship to pocket depth was found. SP containing two endoflagella were the pre-dominating type. On the basis of the dimensions of the diameter, the length, the wavelength and the amplitude, the SP fell into four significantly different morphological groups characterized by 1, 2, 3-8, and more than 8 endoflagella, as the values in general increased from group to group.