Abstract. Twenty-six specific-pathogen-free pigs were fed pure cultures of Treponemu hyodysenteriue. Five untreated pigs were controls. Distribution of this large spirochete in pigs with swine dysentery was shown by the indirect fluorescent antibody technique. Findings by this method were compared with those from dark-field examination of colonic mucosal scrapings and from tissue sections. The cultures caused mucohemorrhagic colitis which by 10 days after inoculation was indistinguishable from the colitis of swine dysentery. Control pigs remained normal. Pigs killed when spirochetes were first seen in their feces had normal colonic mucosa with only a few spirochetes. At the first sign of diarrhea, however, the colonic mucosa was thicker than normal and had many spirochetes. T. hyodysenreriue was confined to regions of hypertrophy and exudation of the large intestine mucosa throughout the course of disease.Culture studies, dark-field microscopy, special staining techniques and electron microscopy have suggested a relationship between T. hyodysenreriue and swine dysentery [14][15][16]. Observational limitations, however, have precluded correlation of the findings with the development of the disease. Culture and dark-field observations place the organisms only in the gross vicinity of the lesion; special staining techniques do not identify organisms specifically, and electron microscopy gives a very selective view of the morphologic relationship between the organisms and the disease. We used specific immunofluorescent staining of T. hyodysenreriue in the affected tissues to correlate the presence of the organisms with the development of the lesions.A mucohemorrhagic diarrhea disease of pigs was shown to be distinct from hog cholera and salmonellosis in 1921 and was named swine dysentery [26]. A large spirochete was associated with the lesions but feeding a mixed culture containing the organism did not produce the disease. A comma-shaped organism also was seen in swine dysentery at that time and was later isolated and named Vibrio coli. This was considered the cause of the disease [9-111. Although V. coli was generally accepted to be the cause of swine dysentery [4,7, 10, 111, this organism has been commonly found in normal pigs [2, 8, 131, and in 490