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Abstract Introduction: Bacterial diarrhea is very common particularly in devoloping countries and is still one of the most causes of mortalities in children.The aim of present study was to identify the most common of bacterial agents causing acute bacterial diarrhea in children under 12 years old and detection of their resistance to antibiotics in patients referred to pediatric word de of academic hospitals of Hamadan. Materials & methods: During two years) 2003-2005(, 610 samples obtained from children under 12 years old with gastroentrotitis were investigated for bacterial cultures, frequency of age, serogrouping of isolates and antibiogram patterns. Polyvalent (I ،II، III, IV) and monovalent anti-sera (055، 0111، 011، 086، 026 ،0125، 0119، 0146، 0128، 0142، 0157) were used for sero-grouping of E. coli (EPEC). Antibiogram tests were also performed by gel-diffusion method of Kirby-Bauer. The data were gathered through a questionnaire and analysed using SPSS software. Findings: Out of the 610 tested samples, 155 cases (25.4%) had positve cultures for intestinal pathogenic bacilli. The most common isolate was Escherichia coli (EPEC) with 105 cases (67.8%) and the lowest isolate was Shigella with 18 cases (11.6%). The most common serogroup of E.coli was 0128 (26.6%) and the lowest serogroup was 0119 (5.7%). The most common serogroups of Salmonella were S.typhi (34.4%) and S.typhimurium and the lowest serogroup was S. para A (3.1%). The most common serogroup of Shigella was S.sonnei (55.6%). The most effective antibiotics against bacteria were ceftriaoxne, nitrofurantoin, imipenem, amikacin and gentamycin. Conclusion: The present study showed that Escherichia coli (EPEC) and Salmonella species are predominant causes of bacterial diarrhea in children under 12 years old in this region. In many other countries, the most common serogroups of E.coli are 0157 and 055, but in our study the serogroup of 0128 was common. Most species showed high resistance to routine antibiotics such as ampicillin, trimethoprim and chloramphenicol.