Strain CS1(T) (T = type strain) is a gram-negative, microaerophilic, urease-positive, spiral-shaped bacterium that was isolated from the gastric mucosa of a cat. Additional strains which possessed biochemical and ultrastructural characteristics similar to those of strain CS1(T) were isolated from the gastric mucosa of cats and dogs. The guanine-plus-cytosine content of the DNA of strain CS1(T) was 42.5 mol%. The 16S rRNA sequences of strain CS1(T), strain DS3 (a spiral-shaped isolate from a dog), and Helicobacter mustelae were determined by direct RNA sequencing, using a modified Sanger method. These sequences were compared with the 16S rRNA sequences of Helicobacter pylori, 'Flexispira rappini', Wolinella succinogenes, and 11 species of campylobacters. A dendrogram was constructed based upon sequence similarities. Strains CS1(T) and DS3 were very closely related (level of similarity, (9.3%). Two major phylognenetic groups were formed; one group consisted of strains CS1(T) and DS3, H. mustelae, H. pylori, 'F. rappini', and W. succinogenes, and the other group contained the true campylobacters. The average level of similarity between members of these two grops was 84.9%. Within the first group, strains CS1(T) and DS3, H. pylori, and H. mustelae formed a cluster of organisms with an interspecies similarity level of 94.5%. The phylogenetic positions of W. succinogenes and 'F. rappini' were just outside this cluster. On the basis of the results of this study, we believe that strains CS1(T) (= ATCC 49179(T)) and DS3 represent a new species of the genus Helicobacter, for which we propose the name Helicobacter felis.