Investigating the cause of an outbreak of mild diarrhoea in a boar rearing unit with about 750 animals, C. coli was found in all diseased animals, i.e. it was isolated from the intestines of culled boars and from faeces samples; often the amount of C. coli was very high. The intragastric application of C. coli to 5 weaning pigs (about 35 kg) resulted in the shedding of large numbers of C. coli via the faeces at days 3 and 5 post infectionem only. The development of the animals was undisturbed during the 17 days of the experiment. Carrying out follow-up investigation, C. coli was isolated at least once per animal from all healthy animals (n = 40) from the 3 farms, delivering young boars to the unit. C. coli is regarded as an opportunistic microorganism in the intestines of swine, able to cause diarrhoea only when combined with infection-promoting factors. In the case presented, these factors were nitrate-enriched water, changes in the composition of feed and the rapid shift from winter conditions to a warm spring.