The proteobacterial genus Helicobacter is composed of gastric species, all of them urease-positive, and enteric species (gastrointestinal, intestinal, hepatic, biliary), some of them urease-positive, others not. Here, we point out that the gastric species are divided in at least two phylogenetic groups, one is homogeneous, clearly separated from the enteric species, and another is forming a tight cluster within the enteric species. This feature is apparent in the phylogeny of the genus as inferred from both the 16S rRNA gene and the alpha-subunit of the urease. Our observation shows that the ability to colonize the gastric mucosa appeared more than once in the history of the genus, and suggests that acquiring this ability may be a relatively simple and punctual process, involving a limited number of genes. Such a process may be the lateral transfer acquisition of a functional copy of the gene ureI which encodes a urea channel activated at acidic pH that is essential for gastric colonization by Helicobacter pylori.