A weakly beta-haemolytic spirochaete, isolate P43/6/78, was isolated from a pig with diarrhoea and found not to fluoresce with a specific fluorescent antiserum to Treponema hyodysenteriae. Pure cultures of this spirochaete were used to inoculate experimental pigs. Diarrhoea, containing clear mucus, and, in one case, blood occurred in four of the eight animals inoculated. Colitis was present in six of the eight inoculated pigs at necropsy. Excess clear mucus and punctate haemorrhages were seen on the colonic mucosa and spirochaetes resembling isolate P43/6/78 were reisolated from the affected mucosa. The feed conversion efficiency and growth rate of affected pigs was reduced when compared with controls. Isolate P43/6/78 differed from T hyodysenteriae in its cultural, ultrastructural, biochemical and antigenic characters. On these grounds, and because of the clinical and pathological syndrome produced, it was considered to belong to a species other than T hyodysenteriae.