Abstract: Objective To detect Dengue virus nucleic acid in blood serum of patients with unknown fever in Dushan and Xing yi of Guizhou,China in the summer and autumns.Inquiry into the sourec,the evolution characteristics and the relation between the base change and virulence,lay the foundation for immuno genicity and immunologic protection of Deng ue virus.Methods Distill Dengue virus DN A by reverse transcriptase polymerase chain reaction(RT-PCR)and detect target cDNA and inoculate C6/36 cells with the blood serum.Results Positive probability of Dengue virus nucleic acid is 98%(35/356) in the blood serum samples and seven samples(five of Dushan and and two of Xingyi)were carr ied through sequence analysis.They are Dengue virus type 2(DEN-2)through sequence analysis and phylogenic tree,with the same source of DEN- 2-44 and DEN-2-43,and the same with DEN-2-M that detected from Aedes albo pictus in Guizhou in 2003.Inoculat ing C6/36 cells with the blood serum,positive probability is 1.6%(6/356)from RT-PCR detection.Conclusion There was Dengue virus infection among the population of Guizhou province,China.