Aims: To characterize a number of Shiga toxin-producing Escherichia coli (STEC) isolates from sheep and to discuss the potential of these isolates as human pathogens. Methods and Results: Twelve different O-groups and seven different H-types were identified by standard serotyping methods. The most common serotypes were O5: NM, O6: H10, O91: NM and O128: NM. Polymerase chain reaction (PCR) was used for the detection of virulence factor genes. Of 102 isolates, 86.3% carried stx(1) and 83% of these were also positive in the stx(1OX3)-specific PCR. stx(2) was carried by 55.9% of the isolates and 77.2% of these were also positive in the stx(2d)-specific PCR. The Vero cell assay showed high toxin production in 70.6% of the isolates. None of the isolates carried eae. Conclusions: The study supports the animal-host relationship suggested in other studies with STEC serogroups O5, O91 and O128 strongly associated with sheep. Most sheep STEC carry stx(1OX3) ( except O91) and the dominating stx(2) variant is stx(2d). One stx profile clearly dominates within a serotype. Significance and Impact of the Study: In spite of the predominance of certain sheep-associated STEC, sheep cannot be excluded as carriers of human pathogenic STEC.