A coinfection of O177:NM and O55:H7 Shiga toxin-producing Escherichia coli (STEC) was identified for a child with acute bloody diarrhea and hemolytic uremic syndrome by using culture and serotype-specific molecular reagents. The profile of O157-related genetic islands revealed that the O55:H7 isolate was highly similar to O157 STEC whereas the O177:NM isolate lacked several fimbrial O islands and non-locus-of-enterocyte-effacement effector determinants. However, both STEC serotypes are known to cause serious disease, and the significant repertoire of virulence determinants in both strains made it impossible to determine their individual contributions to the clinical symptoms.