Abstract: The author chose 45 OTU' s (namely 45 poplar rusts species) from the world, and chose 24 non-redandant characters in Uredinia, Telia, hosts and alternate hosts of these 45 OTU's. The operational basis of numerical taxonomy as presented by Sneath and Sokal. In this study 24 non-redundant equally weighted qualitative or qualitative characters representing 45 OTU's were entered into a basis data matrix, Cluster analysis (UPGMA) and principal component analysis (PCA) were produced from the original matrix. The results of UPGMA and PCA are the same, and show that 45 OTU's are classified into 14 groups. It means there are 14 poplar rusts species in the world. The 14 species are the following:1 M. pruinosae Tranz. 2 M. laricis Hartig. 3 M. pinitorqua Rost. 4 M. pulcherrima Maire. 5 M. magnusiana Wagn. 6 M. rostrupii Wagn. 7 M. abietis-populi S. Imai ex S. Ito 8 M. medusae Thuem. 9 M. abietis-canadensis (Farl,) Ludwig. 10 M. occidentalis Jacks. 11 M. ciliata Barcl. 12 M. microspora Tranz. 13 M. larici-populina Kleb. 14 M. allii-populina Kleb.The present paper gives the difference of some similar species. The author also discuss the affinity of the species and the evolution of some species, and think that it is very important to know all the poplar species and the affinity of them for the poplar introducing and the poplar resistant breeding.

Key words: Poplar rust, Melampsora, Numerical taxonomy, Cluster analysis, Principal component analysis