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A hippurate-negative biovariant of Brachyspira pilosicoli(B. pilosicolihipp-) is occasionally isolated in diarrhoeic pigs in Finland, often concomitantly with hippurate-positive B. pilosicolior Lawsonia intracellularis. We studied pathogenicity of B. pilosicolihipp-with special attention paid to avoiding co-infection with other enteric pathogens. Pigs were weaned and moved to barrier facilities at the age of 11 days. At 46 days, 8 pigs were inoculated with B. pilosicolihipp-strain Br1622, 8 pigs were inoculated with B. pilosicolitype strain P43/6/78 and 7 pigs were sham-inoculated. No signs of spirochaetal diarrhoea were detected; only one pig, inoculated with P43/6/78, had soft faeces from day 9 to 10 post inoculation. The pigs were necropsied between days 7 and 23 after inoculation. Live pigs were culture-negative for Brachyspiraspp., but B. pilosicolihipp-was reisolated from necropsy samples of two pigs. The lesions on large colons were minor and did not significantly differ between the three trial groups. In silver-stained sections, invasive spirochaetes were detected in colonic mucosae of several pigs in all groups. Fluorescent in situhybridisation for genus Brachyspira, B. pilosicoliand strain Br1622 was negative. However, in situdetection for members of the genus Leptospirawas positive for spirochaete-like bacteria in the colonic epithelium of several pigs in both infected groups as well as in the control group. L. intracellularis, Salmonellaspp., Yersiniaspp. and intestinal parasites were not detected. The failure of B. pilosicolistrains to cause diarrhoea is discussed with respect to infectivity of the challenge strains, absence of certain intestinal pathogens and feed and management factors.