The genetic population structure of Rhynchocypris lagowskii steindachneri in three river systems flowing through Yokohama and adjacent areas in southwestern Kanto, central Honshu, Japan, was investigated by examination of nucleotide sequences of the mitochondrial DNA (mtDNA) control region. Although R. lagowskii steindachneri is the only Rhynchocypris species native to the region, the establishment of R. oxycephalus jouyi, which was artificially introduced in the late 1980's, was confirmed in one of the river systems investigated. From the same river system, one of 27 specimens (4%) of R. lagowskii steindachneri was found to have a R. oxycephalus jouyi mtDNA type, implying some genetic disturbance within the former population. Although the R. lagowskii steindachneri populations investigated, distributed within a 40-km geographic range, appeared to belong toa single phylogeographic group, significant differences in haplotype frequencies and diversity levels were found, probably being the result of genetic drift following recent artificial fragmentation and habitat reduction.
