Sylvatic plague (Pasteurella pestis) conditions in S. Africa, U.S.S.R. and the Argentine were reviewed. A plague epizootic among California ground squirrels in 1941 was investigated, and infection was demonstrated in 5 separate localities in Kern County. In each locality the epizootic was evidently of short duration and coincided fairly well with the dispersal of young squirrels (May-June). Plague organisms were found in the fleas Diamanus montanus, Hoolopsyllus anomalus and Echidno-phaga gallinacea; infection was present in fleas before and after that in squirrels. 60 burrows were excavated and plague-infected fleas and carcasses of squirrels were found; the burrows possibly served as a source of infection after the decline of the epizootic in the squirrels. There was evidence that D. montanus, a good plague vector, was displaced at low altitudes by H. anomalus, an apparently poor vector.