Abstract: Phylogenetic relationships among three strains of Tetrahymena (I.e.T.shanghaienisis,T.thermophila Ⅱ and T.thermophila Ⅵ) were investigated using sequences of the first internal transcribed spacer region (ITS-1) of ribosomal DNA (rDNA).Amplified rDNA sequences consisted of 221 bases of the flanking 18S and 5.8S regions,and the entire ITS-1 region (169 or 172 bases).There were 5 variable bases among three strains in the 18S region and 16 variable bases in the ITS-1 region.The affiliation of them were assessed using both Neighbor-joining (NJ) and maximum parsimony (MP) analyses.Variations do exist in the phylogenies created by the two methods.However,the basic tree topologies are consistent.In both the NJ and MP analyses,T.shanghaienisis diverge from the ancestor of Tetrahymena earlier than T.thermophila Ⅱ and T.thermophila Ⅵ,which show that the selfer might be primitive.