It is possible that the outbreaks of plague observed in Roumania and Bulgaria in the 19th century were of local origin because marmots and susliks--known as the main carriers of plague in nature--were widely spread on the territory of these countries. In the subsequent years, marmots died out in consequence of the economic activity of man and the habitats of susliks became sporadic and occurred in small foci. However, data obtained in the last few decades point out the possibility of long-term preservation of plague infection in the populations of rodents occupying small areas of land. The presence of the European suslik in the countries of Eastern Europe and the fleas parasitizing on the rodent, capable of transferring plague infection, permit the authors to raise the question of a possible occurrence of natural foci of plague in Roumania and Bulgaria. Data are given demonstrating the suitability of an extensive epidemiological reconnaissance in these countries.