The microbial flora of stomachs of humans and animals have been the focus of considerable research since the discovery that Helicobacter pylori in humans caused a variety of gastric diseases. Other Helicobacter species have now been isolated from the stomachs of various mammals, including dogs, cats, ferrets, pigs, monkeys, and cheetahs, all of which are associated with various degrees of gastritis in their hosts. Helicobacter species have also been isolated from the intestinal tracts of humans, animals, and birds. Helicobacter species have not only been isolated from intestines of mice but have, in addition, been isolated from diseased livers of inbred and outbred mice, and one of these, H hepaticus is linked to liver tumors. H canis and H pullorum isolated from dogs and chickens, respectively, have also been cultured from feces of diarrheic humans. H fennelliae and H cinaedi are two additional helicobacters primarily associated with lower bowel disease in immunocompromised humans. To date, there are at least 13 formally named species of the new genus, Helicobacter, the majority being proven or suspected gastrointestinal or hepatic pathogens. Several of these helicobacters have zoonotic potential as well. It is predicted that additional Helicobacter species will be identified, and they, too, may be important pathogens in humans and animals.