Abstract
A new species of gudgeon, Microphysogobio rapidus is described from 25 specimens collected from the Naktong River system, Korea. The new species is similar to Microphysogobio yaluensis but is distinguished from the latter and congeners by the combination of the following characteristics: number of papilla on the upper lip, 4.5 scales above the lateral line, short distance from the anus to the origin of the anal fin, wide scaleless area of the abdomen, nuptial colour consisting of a longitudinal green stripe on the lateral side of the body and bright blue colour of the suborbital region and operculum. A key to the species of Microphysogobio from Korea is also included.