An intensive histological search for Helicobacter pylori in gastric biopsy specimens has led to the detection of other spiral shaped bacteria in the human gastric mucosa. The clinical and morphological findings of 39 cases (0.25% of all gastric biopsies performed in the observation period) are reported for 34 patients (87.2%) complaining of upper abdominal discomfort. Five patients (12.8%) had chronic gastritis and 34 (87.2%) chronic active gastritis. The organisms were seen by light microscopy deep in the gastric foveolae and intracellularly. The scanning and transmission electron microscopic findings show bacteria which invade and damage gastric mucosal cells. These organisms are similar to the spiral shaped bacteria found in the stomachs of cats and dogs and non-human primates. In eight patients organisms were not detected after four weeks of treatment with bismuth salts. The disappearance of the organisms coincided with resolution of the chronic active gastritis and the symptoms.