A new species of Physarum discovered in the Trans-Baikal Territory, Russia is described and illustrated. This new species clearly differs from previously known species in several morphological features. The new species and six other species of Physarum (P. bethelii, P. flavicomum, P. galbeum, P. karamanicum, P. oblatum, P. viride) have a very long stalk and threads of capillitium with yellow lime which distinguishes them into a separate morphological group. Sporotheca of the new Physarum are covered with white lime, while the sporotheca of five other species are covered with lime of yellow, orange and green colours. An important distinguishing feature of the new species is the fragile stalk, which breaks off at the top. Sporocarps of the new species were examined by scanning electron microscopy. We present a table of morphological features of Physarum species with long stalks and yellow lime in nodules of threads of capillitium. Based on the sequences of small subunit (18S) of nuclear ribosomal DNA, we reconstructed a phylogeny of the genus Physarum. DNA sequences obtained by us can be used for identification of new species.