Aims: We conducted two short-term studies of cohorts of naturally infected calves to determine the prevalence and concentrations of Escherichia coli O157 shed in faeces. Methods and Results: Two cohorts of calves were sampled; in the first study 14 calves were sampled up to five times a day for 5 days; in the second study a group of 16 separate calves were sampled once or twice a day for 15 days. All cattle within the two cohorts shed E. coli O157 at some point during the respective studies. In 18% of samples, E. coli O157 could only be isolated using immunomagnetic separation after an enrichment period, suggesting concentrations <250 CFU g(-1). The highest concentrations recorded were 6.7 x 10(5) and 1.6 x 10(6) CFU g(-1) for studies 1 and 2 respectively. Conclusions: Persistent, high shedders (shedding >10(3) CFU g(-1)) were evident in both studies but, in the majority of calves, the pathogen was isolated intermittently. Significance and Impact of the Study: The variable patterns of shedding have important implications for the design of appropriate sampling protocols and for gaining meaningful estimates of parameters used in mathematical models of transmission.