A system for the differentiation of Yersinia enterocolotica strains and Yersinia enterocolitica-like strains is introduced using the biochemical and O-serological characters of the strains and their sources. Seven subgroups are proposed. I--III are indole-negative strains. IV include strains which are indole-positive but lack the ability to split aesculin and to ferment salicin. These strains mostly belong to serotype 0:8 and are found in the USA or in Canada. In subgroups V--VII the strains are also indole-positive but they can also ferment salicin and split aesculin. Subgroup V strains cannot ferment rhamnose. Subgroups VI and VII include the strains which ferment rhamnose, and in subgroups VII, to which strains with serotype 0.17 belong, melibiose is also often fermented. Using cross-agglutination studies it has been possible to make a subdividing of the serotypes 0:4, 0:5, 0:6 and 0:7.