A field isolate of Brachyspira pilosicoli, the aetiological agent of porcine intestinal spirochaetosis, was inoculated by stomach tube into six 6-week-old pigs. All animals developed loose to watery faeces and were killed successively on days 8,114 or 17 post-inoculation (day 17 being the end of the study). Mild mucosal reddening and flecks of pus characterized the gross lesions, while diffuse, catarrhal colitis was revealed microscopically in all animals. Intestinal spirochaetosis with moderate to densely packed end-attached B. pilosicoli organisms was revealed extensively on the mucosal surface of the large intestines by light microscopy and fluorescent in situ hybridization. This study is the first to report extensive intestinal spirochaetosis in pigs challenged with B. pilosicoli.