Onychostoma krongnoensis and O. dongnaiensis are the first records of the genus in the upper Srepok River
and middle Dong Nai River on the Langbiang Plateau, southern Vietnam. Among species of the Mekong group, O.
gerlachi occurs the furthest north. This species could be the ancestral lineage of the Mekong clade. Despite the lack
of O. fusiforme in our molecular data, phylogenetic analysis and geographical distribution suggest that this lineage
may have migrated towards the southern Mekong basin. The geological event that separated the Dong Nai dranage
from the Mekong drainage would have isolated O. dongnaiensis from O. krongnoensis. Detailed and
comprehensive surveys to gain a better understanding of geographical distributions, and molecular phylogenetic
analysis for Onychostoma in the Mekong basin are essential.