The phylogenetic relationships of four ciliate genera (Urostyla, Euplotes, Stylonychia and Pseudokeronopsis), which also are the important environment inspection species, were analyzed by the comparison of small subunit ribosomal RNA gene sequences. Euplotes appeared as an early branching group whose divergence from the hypotrichous line at a deep level was strongly supported by parsimony and matrix analyses. The analyses also supported the hypothesis that there were closely relationship between species in Urostyla and Holosticha. The sibling species Stylonychia mytilus and S. lemnae could be separated by the evolutionary analyses. Furthermore, Pseudokeronopsis rubra had relatively more closely relationship with the species in Holostichidae than that in Urostylidae based on the evolutionary distance value.