Objective:To survey the prevalence severe diarrhea arising from these bacteria in children under5 years old inMarvdasht.Methods:In this study faecal sample from615 children aged <5 years old who were hospitalized for gastroenteritis inFars hospitals inIran were collected and then enriched inEscherichia coli(E. coli) broth and modified tryptone soy broth with novobiocin media.Fermentation of sorbitol, lactose and β-glucoronidase activity of isolated strains was examined byCT-SMAC,VRBA and chromogenic media respectively.Then isolation ofE. coli O157:H7 have been confirmed with the use of specific antisera and with multiplexPCR method presence of virulence genes including: stx1,stx2,eaeA,hly has been analyzed.Results:E. coli O157:H7 was detected in7(1.14%) stool specimens.A significant difference was seen between detection rate of isolated bacteria from age groups18-23 months and other age groups(P=0.004). Out of considered virulence genes, only1 of the isolated strains(0.16%) the stx1 andeaeA genes were seen and also all isolated bacteria had resistance to penicillin, ampicillin and erythromycin antibiotics.Conclusions:We found that children <2 years of age were at highest risk of infection withE. coliO157:H7.Regarding severity ofE. coliO157:H7 pathogenesis, low infectious dose and lack of routine assay for detection of these bacteria in clinical laboratory, further and completed studies on diagnosis and genotyping of thisE. coliO157:H7 strain has been recommended.