Helicobacter mustelae, isolated from the stomachs of adult ferrets, appears to have a world-wide distribution. Ferrets are colonized with H. mustelae at a young age, usually 5-6 weeks; in our experience 100% of adult ferrets are colonized in both the antrum and the fundus. Gastric infection correlates with elevation of serum IgG antibodies to H. mustelae. In the oxyntic mucosa the presence of superficial gastritis coincides closely with the presence of H. mustelae. In the distal antrum the organism is associated with chronic inflammation occupying the full thickness of the mucosa. In addition to lesions seen in the distal antrum, focal glandular atrophy and regeneration are noted in the proximal antrum and transitional mucosa. Antibiotics used to eradicate Helicobacter pylori in humans are also effective in eliminating H. mustelae from ferrets. H. mustelae-free ferrets do not become colonized with H. pylori when challenged orally; however, sparse colonization follows oral inoculation with a related gastric organism, "Helicobacter felis." Controlled studies of the pathophysiology of gastroduodenal disease induced by Helicobacter species can be performed in H. mustelae-infected ferrets and their H. mustelae-negative counterparts.