The genomic structure and the restriction maps were studied in 24 Brucella strains isolated from marine mammals. From SpeI restriction profiles, the strains could be ascribed to three clonal groups, each corresponding to a specific host. Cross contamination between exclusively terrestrial and exclusively marine hosts is unlikely suggesting the divergence of the different species of the genus Brucella which may have taken place 60 million years ago, concomitant with the radiation of their mammalian hosts (Artiodactyla) from other mammalian orders.