A new microaerophilic, spirally curved, rod-shaped bacterium was isolated from the gastric mucosa of a pigtailed macaque (Macaca nemestrina). The gram-negative cells of this bacterium are oxidase, catalase, and urease positive and strongly resemble Helicobacter pylori (Campylobacter pylori) cells. Like H. pylori, this organism does not metabolize glucose, does not reduce nitrate or produce indole, does not produce H2S from triple sugar iron agar, does not hydrolyze hippurate or esculin, and does not grow in the presence of 1% glycine, 1.5% salt, or 1% bile. Also like H. pylori, it is resistant to nalidixic acid and susceptible to cephalothin. However, unlike H. pylori, the colorless colonies are flat and have irregular edges. This organism has a unique cellular fatty acid composition, forming a new gas-liquid chromatography group, group K, and a distinctive DNA content (24 mol% guanine plus cytosine). It exhibits less than 10% DNA-DNA homology (as determined by the nylon filter blot method at 65 degrees C) with other members of the genus Helicobacter. Although the levels of DNA relatedness between previously described Helicobacter species and the new organism are low (less than 10%) and the difference in guanine-plus-cytosine content is large (24 versus 36 to 41 mol%), the genus Helicobacter is the only genus in which it is logical to include the organism at this time. We propose that our single strain represents a new species, Helicobacter nemestrinae, and we designate strain T81213-NTB (= ATCC 49396) as the type strain.