AIM--To study the ultrastructural appearances of Helicobacter pylori in antral and duodenal biopsy specimens and its relation with the epithelial cells. METHODS--Endoscopically obtained antral and duodenal biopsy specimens were examined using transmission electron microscopy and freeze fracture analysis. RESULTS--Most bacteria looked curved, but in the duodenal bulb coccoid bacteria were relatively common. Bacteria were often found around intercellular junctions. freeze fracture examination indicated abnormalities of the tight junction complexes in patients with H pylori infection. In many biopsy specimens bacteria were seen closely attached to the epithelial cell membrane by different forms of adhesion. In addition to what looked like intracytoplasmic penetration by bacteria, several examples of genuine penetration were observed. CONCLUSION--H pylori is commonly found adhering to epithelial cells. Occasionally, H pylori may also penetrate cells. These features may contribute to the pathogenic action of the organism.