A medium (»Y« medium) is described, which was more efficient for the isolation ofYersinia enterocoliticafrom experimentally infected faecal specimens than desoxycholate‐citrate, McConkey, lactose‐sucrose‐urea (LSU) agar, and Yersinia selective medium (Wauter's medium). The »Y« medium consists of casein hydrolysate and peptone serving as carbon and energy sources. A high selectivity is achieved by its contents of sodium oxalate and bile salts. The oxalate suppresses growth of gramnegative rods, including members of the familyEnterobacteriaceaeand ofPseudomonasspp., while the bile salts inhibit growth of gram‐positive bacteria. In the few instances coliform rods grew on the »Y« medium, they could easily be distinguished by their fermentation of lactose, included in the medium, and the fact that colonies of organisms were surrounded by an opaque zone of precipitated bile salts. The most optimal condition for the isolation ofY. enterocoliticafrom stools was achieved at incubation of the »Y« medium at 2