【Abstract】 Objective To understand the objective level of the bacterial embolus forming of the fleas （Citellophilus tesquorum altaicus） after its hibernation.  Methods C. tesquorum altaicus hungered for 3 d were fed with the dying mice infected by Yersina pestis on September 28, 2006, and no blood-sucking fleas were removed. Four hundred and thirteen infected fleas were put into the deepfreez at 2 to 4 ℃ and relative humidity （RH） 75%-85%. The infected fleas were taken out from refrigerator on April 6, 2007, and sucked blood from non-infected mouse one time. Then they were put into the incubator at 20 to 22 ℃ and RH 75%-85%. They were fed for 0.5-1 h at one day intervals.  The  forming  of  bacterial  embolus  was  observed  by   microscope.  Results There  were 235 C.tesquorum altaicus alive after hibernation, and nine fleas formed the bacterial embolus, which the forming rate of bacterial embolus was 3.83% （9/235）. Conclusion The forming rate of bacterial embolus of the fleas carrying the plague to over-winter was obviously lower than that of the fleas in the active stage （13.8%）, which might be related to fleas diapause at the disadvantage environment and the situation that some fleas got rid off plague or the  plague quantity carried by fleas decreased, less than 10² to 10⁴.