Being inoculated parenterally various microbes cause the fleas X. cheopis a stable infection which, as a rule, is preserved in experimental insects to the end of life. Reproducing intensively Listeria monocytogenes and Salmonella typhimurium caused the death of all ectoparasites in 3 to 5 days. The increase in abundance of Escherichia coli, Yersinia pseudotuberculosis, Y. enterocolitica and vaccine strains of Y. pestis "EV" and Francisella tularensis went on gradually and infected fleas lived up to 15--20 and more days.