AbstractGene introgression usually results from spontaneous hybridization occurring among closely related species in sympatric populations and thus has great impact on the genetic structure of natural populations. This phenomenon has recently become one of the most active subjects in study of evolutionary and conservation biology. In the present study, eight chloroplast microsatellite (cpSSR) primers were used to examine genetic diversity, population genetic structure and hybridization introgression among seven Actinidia species with overlapping distributions in transitional regions ranging from the western China plateau to the middle-eastern China hilly mountains. The results showed: (1) A total of 42 haplotypes designated by 18 alleles at six polymorphic loci were detected across 655 individuals of the ten sympatric populations sampled. Although a reticulate evolutionary relationship was revealed among cpSSR haplotypes, there existed a strong species-specific tendency. (2) The intra-specific genetic diversity was generally high but varied greatly among different species. A. fulvicoma var. lanata showed the highest genetic diversity (P = 62.50%, h_T = 0.173, H_T = 0.897) whereas A. deliciosa exhibited the lowest (P = 37.5％, h_T = 0.041, H_T = 0.516). (3) High level of inter-population genetic differentiation (G_ST and F_ST varied from 0.319 to 0.780 and 0.40 to 0.695, respectively) and limited gene flow (N_m ranged from 0.219 to 0.747<1) were detected among the different species, of which A. deliciosa showed the highest inter-population genetic differentiation (G_ST = 0.780, F_ST = 695). (4) No significant difference was found between Gst (unordered alleles) and Nst (ordered alleles), sug-gesting a lack of phylogeographical structure for most Actinidia species. This is consistent with the Mantel test which revealed no significant relationship between genetic distance and geographical distance. (5) Inter-estingly, a relatively high level of inter-specific genetic differentiation was detected among species in sym-patric populations (F_ST ranged from 0.476 to 0.990), except for two populations (HW and GZ) of the A. chinensis/A. deliciosa complex. The result was also well in accordance with UPGMA analysis, which re-vealed that the populations tended to cluster together into the same species. Further analysis indicated that the A. chinensis/A. deliciosa complex was probably highly influenced by both shared ancestral chloroplast polymorphism and recent hybridization introgression. However, hybridization introgression rarely occurred between less related species, suggesting homoplasy events may have also played an important role in shaping phylogenetic relationships in the genus. The results have important implications for understanding population genetics and evolutionary patterns in the genus Actinidia and provide baseline data for formulating conserva-tion and management strategies.