Aims: To determine the prevalence and concentration of Escherichia coli O157 shed in faeces at slaughter, by beef cattle from different production systems. Methods and Results: Faecal samples were collected from grass-fed (pasture) and lot-fed (feedlot) cattle at slaughter and tested for the presence of E. coli O157 using automated immunomagnetic separation (AIMS). Escherichia coli O157 was enumerated in positive samples using the most probable number (MPN) technique and AIMS and total E. coli were enumerated using Petrifilm. A total of 310 faecal samples were tested (155 from each group). The geometric mean count of total E. coli was 5 x 10(5) and 2.5 x 10(5) CFU g(-1) for lot- and grass-fed cattle, respectively. Escherichia coli O157 was isolated from 13% of faeces with no significant difference between grass-fed (10%) and lot-fed cattle (15%). The numbers of E. coli O157 in cattle faeces varied from undetectable (<3 MPN g(-1)) to 1.1 x 10(5) MPN g(-1). Twenty-six (67%) of 39 O157 positive faeces had <10 MPN g(-1) and three (8%) had counts between 10(3)-10(5) MPN g(-1). There was no significant difference between concentrations of E. coli O157 in the faeces of grass-fed or lot-fed cattle. Conclusion: The prevalence and numbers of E. coli O157 in the faeces of cattle at slaughter were not affected by the production systems evaluated in this study. Significance and Impact of the Study: Information on the prevalence and numbers of E. coli O157 can be used for formulating intervention strategies and in quantitative risk assessments.