An experimental infection was induced in the European ground squirrel (Citellus citellus) by intraperitoneally injecting of 1 ml. of microbial suspension, containing 1 x 10(9) CFU (colony forming units) of Yersinia pseudotuberculosis, serotypes I and III. The infectious process was dynamically followed up from the 1st up to the 30th day, respectively. Account was rendered to the general condition, the internal body temperature and the mortality rate. The phagocytic activity of alveolar macrophages (aMa) and polymorphonuclear leucocytes (PMNL), isolated from the experimental animals, was also determined, as well as the degree of bacterial dissemination into the viscera and the histomorphological changes that found in the parenchymal organs. These investigations confirm the fact that the bacterial agent has a broad spectrum of alterations in the organism of the host--the European ground squirrel (Citellus citellus) in which the pseudotuberculosis develops as a generalized infection, especially pronounced in the animals that were infected with the serotype III.