The Escherichia coli Shiga toxin (Stx) producing strains may cause medical conditions that range from diarrhea to hemolytic uremic syndrome; life-threatening conditions worldwide. Shiga toxin-producing E. coli (STEC) strains isolated from patients who usually possess, in addition to one or more stx genes, the eae gene, encoding adhesin intimin. However, a subset of STEC strains associated with human disease lack eae. One of the most common among these is the serogroup O91. This paper reports on the isolation and description of the genophenotypic characteristics from a bovine-derived E. coli O91:H21 strain in a slaughterhouse dependent from Cattle Direction of Mendoza Province, during 2007. Key words: E. coli O91:H21, hemolitic uremic syndrome (HUS), diarrhea, cattle.