Experimentally infected fleas of X. cheopis are capable of preserving two strains of Y. enterocolitica. One of the strains was found to belong to the 3rd, the other to the 9th serotype. The both strains actively multiplied and accumulated in great amount (up to several millions of microbial cells) in ectoparasites which were maintained at 6 to 8 degrees C. At a temperature of 18 to 21 degrees C some part of fleas gradually became free from the agent that was favoured by their periodical feeding or constant maintenance on the host. In insects infected with the strain of the 3rd serotype the decrease of the infected individuals carried out quicker and was accompanied by the reduction in the number of microbes. The strain of the 9th serotype preserved more steadily and was found in many individuals in amount exceeding the initial level.