Background. Barrett's esophagus ( BE) is a complication of chronic gastroesophageal reflux disease, in which patients are at greatly increased risk of esophageal dysplasia and adenocarcinoma. Over the past 2 decades, there has been an increase in the incidence of both BE and adenocarcinoma; however, the involvement of microorganisms in BE is uncertain. The aim of this study was to characterize microbial communities in esophageal aspirate specimens and on distal esophageal mucosal samples from patients with BE. Methods. Biopsy and aspirate specimens were obtained by endoscopic examination from 7 patients with BE and 7 control subjects without BE. Samples were cultured under aerobic, anaerobic, and microaerophilic conditions for yeasts and bacteria, including Helicobacter pylori. Bacterial isolates were identified by 16S ribosomal RNA gene sequencing. Fluorescence microscopic examination was also used to determine the spatial localization of these organisms on mucosal surfaces. Significant colonization was detected in 6 patients with BE and in 4 control subjects. Results. Overall, 46 bacterial species belonging to 16 genera were detected, with 10 species being common in both groups. Both aspirate and biopsy samples from patients with BE contained complex populations of bacteria. Uniquely, high levels of Campylobacter species ( Campylobacter concisus and Campylobacter rectus), which have been linked to enteritis, periodontal infections, and tumor formation in animals, were found in 4 ( 57%) of 7 patients with BE but in none of the control subjects. Microscopic examination revealed that bacteria on mucosal biofilms often occurred in microcolonies. Conclusions. The occurrence of nitrate-reducing Campylobacter species in patients with BE may suggest that there is a link in either the initiation, maintenance, or exacerbation of disease processes leading to adenocarcinoma formation.