The effects of five different diets on the experimental infection of pigs with a Danish field isolate of Brachyspira pilosicoli were investigated. The diets tested were a pelleted and a non-pelleted standard diet based on wheat and barley, the standard diet supplemented with 2 per cent lactic acid, a fermented liquid feed and a diet based on cooked rice. Two trials were conducted, each with six groups of six pigs; in each, two of the groups were fed the standard diet. One of these groups and the other four groups were challenged after two weeks on the diets and euthanased four weeks later. The clinical signs of B pilosicoli infection varied from loose stools to watery, mucoid diarrhoea. The group fed the rice diet excreted B pilosicoli in their faeces for a significantly shorter period than the group fed the standard diet (P < 0.01), and fewer of them excreted the organism (P < 0.05). All the pigs fed the pelleted diet excreted B pilosicoli in their faeces, and significantly more of them showed clinical signs of disease than the pigs fed the standard diet (P < 0.05). The fermented liquid feed and the diet containing lactic acid had no significant effect on the excretion of B pilosicoli or on the numbers of pigs showing clinical signs of disease.