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The Rifr mutant of Y. pseudotuberculosis, capable of producing pure cultures in media with a high content of rifampicin, has been used for an accurate quantitation of this microorganism in various kinds of natural (nonsterile) soil in controlled laboratory and field experiments. The main biological characteristics of the mutant have been identical to those of the parent strain. The first experiments have shown that the initially high concentration of Y. pseudotuberculosis in the soil gradually decreases in 2 months. The share of this microorganism in the natural microflora of the soil seems to be rather small, which probably explains the cause of low indices of spontaneous contamination of the soil in nature.