Escherichia coli O157 shedding in 14 cattle herds was determined by faecal culture at intervals of approximately 1 month for up to 13 months. The overall prevalence was 1·0% (113/10832 faecal samples) and 9 of the 14 herds were detected as positive. Herds positive 2 years previously (n=5) had a higher prevalence of positive cattle (median=1·9%) than herds which had been negative on a previous sampling (n=8, median=0·2%). Weaned heifers had a higher prevalence (1·8%) than did unweaned calves (0·9%) or adults (0·4%). For all herds the highest prevalence occurred in the summer months, which resulted in most of the positive faecal samples being collected on a minority of sampling visits.