Objective:To survey the prevalence severe diarrhea arising from these bacteria in children under 5 years old in Marvdasht.Methods:In this study faecal sample from 615 children aged <5years old who were hospitalized lor gastroenteritis in Fars hospitals in Iran were collected and then enriched in Escherichia coli(E.coli) broth and modified tryplone soy broth with novobiocin media,fermentation of sorbitol,lactose and β— glucoronidase activity of isolated strains was examined by CT—SMAC,VRBA and chromogenic media respectively.Then isolation of E.coli O157:H7 have been confirmed with the use of specific antisera and with multiplex PCR method presence of virulence genes including:xtx_1.stx_2,eae.A.hly has been analyzed.Results:E.coli O157:H7 was detected in 7(1.14%) stool specimens.A significanl difference was seen between detection rale of isolated bacteria from age groups 18-23 months and other age groups(P=0.004).Out of considered virulence genes.only 1 of the isolated strains(0.16%)he stx,and eaeA genes were seen and also all isolated hacleria had resistance to penicillin,ampicillin and erythromycin antibiotics.Conclusions:We found thai children < 2 years of age were at highest risk of infection with E.coli O157:H7.Regarding severity of E.coli O157:H7 pathogenesis,low infectious dose and lack of routine assay for detection ol these bacleria in clinical laboratory,further and completed studies on diagnosis and genolyping of this E.coli O157:H7 strain has been recommended. Objective: To survey the prevalence of severe diarrhea arising from these bacteria in children under 5 years old in Marvdasht. Methods: In this study faecal sample from 615 children aged <5 years old who were hospitalized lor gastroenteritis in Fars hospitals in Iran were collected and then enriched in Escherichia coli (E. coli) broth and modified tryplone soy broth with novobiocin media, fermentation of sorbitol, lactose and beta-glucoronidase activity of isolated strains were examined by CT-SMAC, VRBA and chromogenic media. O157: H7 have been identified with the use of specific antisera and with multiplex PCR method presence of virulence genes including: xtx_1.stx_2, eae.A.hly has been analyzed. Results: E. coli O157: H7 was detected in 7 (1.14 %) stool specimens. A significant difference was seen between detection of rale of isolated bacteria from age groups 18-23 months and other age groups (P = 0.004). Out of considered virulence genes.only 1 of the isolated strains (0.16% ) he stx, and eae A genes were seen and also all isolated hacleria had resistance to penicillin, ampicillin and erythromycin antibiotics. Conclusions: We found thai children <2 years of age were at highest risk of infection with E. coli O157: H7.Regarding severity of E. coli O157: H7 pathogenesis, low infectious dose and lack of routine assay for detection ol these bacleria in clinical laboratory, further and completed studies on diagnosis and genolyping of this E. coli O157: H7 strain has been recommended.