| Summary: | The aquatic plant Sagittaria aginashi, which is an endangered emergent species distributed in South Korea, Japan, and the Russian Far East, has an unclear phylogeny and genetic structure. Phylogenetic trees of Sagittaria were constructed based on one nuclear ribosomal DNA (nrDNA) region and on three chloroplast DNA (cpDNA) regions. S. aginashi was included in a clade comprising three endemic Chinese Sagitarria species characterized by the presence of small tubercles at the base of the axils. Furthermore, phylogenetic trees based on cpDNA sequences indicated that S. aginashi was more closely related to S. tengtsungensis than to two other species. S. aginashi exhibited little genetic variation, suggesting that this species had rapidly spread in these areas. With regard to factors associated with the rapid spread, we considered that multiple dispersal methods were involved, namely hydrochory via tubercles, endozoochorous long-distance dispersal, and anthropogenic impacts of rice farming on S. aginashi dispersal. Keywords: Aquatic plant, Internal-transcribed spacer, Sagittaria lichuanensis, Sagittaria potamogetifolia, Sagittaria tengtsungensis
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