Flea-borne bubonic plague cannot exist in epidemic proportions when the mean monthly ambient temperature exceeds 27.5°C. At temperatures in excess of 27.5°C, many fleas do not become blocked with Pasteurella pestis (Yersinia pestis), thus decreasing the transmission potential. The blocking process is associated with fibrin formation from ingested blood by a P. pestis coagulase and a trypsin-like enzyme associated with the stomach of Xenopsylla cheopis. Fibrin provides the matrix for the lodgement of P. pestis in the flea proventriculus that is required for the blocking process. When temperatures are elevated above 27.5°C, fibrin is rapidly destroyed by a fibrinolytic factor of P. pestis and the trypsinlike enzyme. Deprived of fibrin matrices, P. pestis are rapidly eliminated from the flea proventriculus. There is an inverse straight-line relationship between the activities of the fibrinolytic factors and the blocking of fleas with P. pestis. These observations provide a physiological explanation for the spontaneous decline of plague epidemics observed with the advent of hot weather.