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The palaemonid shrimp Palaemon sinensis, a species native to China, was first discovered as an established non-native species in Japan in 2005, and it has since been reported in 25 of 47 prefectures. To genetically elucidate that P. sinensis was introduced to Japan from China via importation as a live fishing bait species, this study analyzed partial nucleotide sequences of the mitochondrial 16S rRNA gene of 165 specimens collected in 7 geographical regions in Japan (Tohoku, Kanto, Chubu, Kinki, Chugoku, Shikoku and Kyushu), and from 18 imported specimens from China purchased at a fishing store. All sequences were classified into 11 haplotypes, which included one widespread haplotype shared by more than half of the P. sinensis samples. Almost all of the samples of imported specimens possessed the dominant haplotype. Analysis of molecular variance showed no genetic differentiation between the Japanese local and imported samples or among regions in Japan. The overall results suggest that P. sinensis was introduced through human-mediated dispersal; furthermore, the haplotype distributions of P. sinensis established in Japan reflected the intensity of the species' importation from central and northern China. This is the first study to document the genetic structure of non-native P. sinensis in Japan. [ABSTRACT FROM AUTHOR]