A one year etiological survey of diarrhea in infants aged less than two years who were attending three hospitals in Baghdad. A total of (600) cases of diarrhea and (100) aged matched controls were investigated. The study involved (400) infants selected for previously untreated diarrhea, and (200) without regard to prior treatment. The mean age of all cases was (9.7) months. Males accounted for higher percentage of all diarrheal cases. The study identified the etiological agents and risk factors affecting the incidence of diarrhea among infnnts. In addition, one of these causative agents have been studied on the molecular level to determine its virulence factors. Furthermore, itttoropathogenic bacteria were exposed to colony hybridization to detect the .presence within their genomic content of DNA encoding heat-labile (LT) entorotoxin Biological and socioeconomic factors influencing diarrhea prevalence ratos, Studied. The following variables were significantly associated with an increased risk of diarrhea: young age (0-6) months; lack of breastfeeding; maternal age (less man thirty years); uneducated mothers; big families. The predominant fecal flora was Bifidobacterium and Lactobacillus in foces from breastfed infants (mostly healthy). This contrasted with coliforms, Bacteroides and Closlridium predominance in bottlefed infants (mostly patients). Enteropathogens were identified in (48.7%) of the inpatients and (32.8%) Of the outpatients. Altogether, bacterial viral and parasitic agents were found in (48%, 20.5%, 20.3%), respectively. (13.3%) harbored more than one entiropathogen, and no pathogens were detected in (18.5%). The bacteria most freqently encountered were enteropathogenic £. co//(11.8%) enterotoxigenic R coll (11.3%), enteroaggregative E. coll (8.7%) and Clostridium difficile (3.8%) while Enlamoeba histolylica (13.2%) and Cryptosporidium parvum (4,2%) were most common among the parasites. Bloody diarrhea was associated with C. difficile and E. histolytica. Whereas vomiting was associated with ETEC and rotavirus. The pathogens with higher median duration of diarrhea were EAEC (14 days) and EPEC (11 days). The activity of antimicrobial agents were tested against Aeromonas spp.. All isolates were sensitive to gentamycin, nalidixic acid, rifampicin and streptomycin, and resistant to ampicillin and cephalexin. Multiple drug resistance was common (54.5%). The plasmid content of Aeromonas isolates was also investigated. It was found that all strains had a common plasmid. Transformation experiment conducted with one Aeromonas isolate showed that ampicillin and cephalexin resistance and hemolysin production were encoded by plasmid. A total of (328) enteropathogenic bacteria isolated from infants with diarrhea were examined by hybridization with biotinylated DNA probe for the gene that coded for production of heat-labile enterotoxin. (68) isolates of LT-ETEC were found, while (36) of the LT-producing isolates were Salmonella typhimurium, Campylobacter spp., Aeromonas spp. and Shigella spp. None of the EAEC and enterohemorrhagic E. coli isolates hybridized with the LT-probe.