We found spiral bacteria (non-Helicobacter pylori, SB) in gastric biopsies of 5 patients corresponding to an incidence of 0.3%. The bacteria were found on the surface of the gastric mucosa and in part tightly packed within the crypts. Contrary to Helicobacter pylori, most of them had no direct contact to the surface and crypt epithelium. They are distinctly coiled, 3.6-5.5 microns in length and on average 0.5 microns thick. Ultrastructural studies revealed sheathed flagella at each pole. In one case the bacteria displayed periplasmic fibrils in pairs as also described in cultures of SB from cats' stomachs. In all 5 cases there was histological evidence of inflammation of the gastric mucosa, i.e. one acute diffuse gastritis, one case of granulomatous and three of slight to medium grade chronic gastritis. Biopsies of 2 patients showed a positive urease reaction in the CLO test. Morphologically very similar SB occur as commensals in the stomachs of various animals, in particular dogs and cats. We investigated the stomachs of four dogs and four cats and found all to be infested with SB. The bacteria were found not only on the surface of the mucosa and in crypts, but within the glands of the corpus and antrum and often also within parietal (oxyntic) cells. Yet despite bacterial colonization there was no evidence of gastritis in dogs. However, all of the cats' stomachs showed slight to medium grade chronic gastritis. Cultivation of SB has not been successful so far, with the exception of cats' stomachs. Since the germs have been defined only morphologically, the question as to how close the relationship is among SB of various origins must for the time being remain unanswered. Furthermore, species-specific pathogenicity and the possibility of contagion from animal to man has not yet been clarified.