To investigate if there is a reservoir of Escherichia coli O157 in Norwegian cattle, faecal samples from 197 cattle herds were screened for E. coli O157 by the use of immunomagnetic separation (IMS) and PCR during the 1995 grazing season. Six E. coli O157:H-isolates were detected in two herds, one isolate in one and five in the other. The isolates carried the stx1, stx2, and eae genes, and a 90 MDa virulence plasmid. They were toxinogenic in a Vero cell assay. From 57 other herds, 137 faecal samples were positive for stx1 and/or stx2 genes detected by PCR run directly on IMS-isolated material. Among these samples, stx2 were the most widely distributed toxin encoding genes. No difference was found among milking cows and heifers in the rate of stx1 and/or stx2 in positive samples.