It has been reported that several floras and faunas in rivers are genetically impaired by artificial structures such as dams and cities because of the fragmentation and the reduced heterogeneity of their habitats. However, the investigations on genetic diversities of Trichoptera in rivers have been few to date. In this study, therefore, the genetic investigation of Hydropsyche orientalis was conducted at 13 stations in the Natori River System in Miyagi Prefecture, Japan from Dec. 2000 to May 2001. The Okura and Kamafusa dams, and the Sendai urban area are located upstream and downstream of the rivers. The DNA polymorphisms of Hydropsyche orientalis collected at the 13 stations were analyzed with the appropriate primer, OPA-11, for calculating the genetic diversity index. The result of statistical analysis of the index showed a significantly higher level of genetic diversity of Hydropsyche orientalis in the natural area than in the areas around the dams. Moreover, the effect of urbanization on its genetic diversity was indicated based on the estimation of its migration from the results of the cluster analysis of the DNA polymorphisms.