In the winter of 1978-1979, the presence of Clostridium botulinum was studied on a cattle farm, on which botulism caused by feeding the animals contaminated brewers' grains occurred in 1977. Cl. botulinum type B, the cause of mortality among cattle at the time, was detected in grass silage prepared in 1978. This organism was not detectable in a grass silage pit dating from 1977 and made prior to the outbreak of botulism. Investigations showed that proteolytic types of C. botulinum having grass as the substrate may produce large amounts of toxin. The production of toxin in grass silage pits may, however, be prevented by a low pH in conjunction with low water activity. The present study showed that the faeces of cattle were also contaminated with Cl. botulinum type B. The degree of infection ran parallel with the degree of contamination of silage feeding in these cases.