In this work the research results on development of selective biphase medium for detachment and bacterial identification of microorganism Y. ruckeri are shown. The selective medium consists of 2 phases: 1)liquid, on the basis of meat-peptone broth, sodium azide (NaN₃) and sodium dodecyl sulphate; 2) dense, including agar, maltose, trypton and bromthymol blue. It was established that substrate on the basis of sodium azide and dodecyl sulphate admit growth of Y. ruckeri on dense phase only under condition of weakening NaN₃ with 0,08%, based on its molecule migration into agar. During the test on specificity, engineered biphase medium showed presence of adequate stability of microorganisms of Yersinia species to inhibitory environmental components. Bacteria Y. ruckeri , Y. enterocolitica, Y. pseudotuberculosis species showed good growth even in weak concentrations, but it was not showed by used I experiments bacteria of grampositive and gram-negative groups, such as Staphylococcus aureus, Bacillus cereus, Escherichia coli, Flavobacterium psychrophilum, Аeromonas hydrophila. So the use of this selective medium is entirely sufficient for primary differentiation carrying out of bacteria Yersinia species from other gram-positive and gram-negative microorganisms found in fish water area. For further differentiation of Yersinia ruckeri, cultures it is enough to cultivate cultures, obtained on experimental medium, on Giss medium with arabinose 48 hours at the temperature of 260С. Bacteria Y. species doesn’t ferment arabinose, in contrast to other bacteria Yersinia , which allows to make clear specific belonging of studied cultures.