Deoxyribonucleic acid (DNA) relatedness among members of the genus Yersinia and the biochemical reactions of these organisms were compared. These data were used to reassess speciation within the genus Yersinia, and three new species were proposed. These are Yersinia ruckeri sp. n., formerly called the redmouth bacterium; Yersinia intermedia, sp. n., formerly a rhamnose-positive, raffinose-positive, melibiose-positive biogroup within Yersinia enterocolitica; and Yersinia frederiksenii sp. n., formerly a rhamnose-positive, raffinose-negative, melibiose-negative biogroup within Y. enterocolitica.