Brachyspira pilosicoli (formerly Serpulina pilosicoli) causes swine spirochaetosis and can also be isolated from human faeces, although its role in human disease remains unclear. The genetic and biochemical variations amongst 19 isolates of human spirochaetes from five different countries were evaluated and compared to those found amongst swine isolates of B. pilosicoli. All isolates were negative for P-glucosidase and all but one were positive for hippurate hydrolysis, which are characteristics typical of B. pilosicoli. The isolates showed variation in indole production and agalactosidase and a-glucosidase activity, other characteristics which can be used to identify B. pilosicoli. The DNA sequences of part of the 165 rRNA gene differed from each other and from that of B. pilosicoli by 0-3 bp out of 283 bp.It is concluded that there is considerable variation amongst human intestinal spirochaetes. Since few of the isolates reported here match the current criteria for B. pilosicoli, it is concluded that this species is more heterogeneous than previously appreciated. However, it cannot be excluded that some isolates may belong to uncharacterized related BrachyspiralSerpulina species.