Distribution of a monogenean helminth Microcotyle sebastis on the gills of cultured Korean rockfish (Sebastes schlegeli) was investigated with regard to gill arches, sides of gill hemibranches (anterior or posterior), and their sections (dorsal, medial and ventral). M. sebastis has a significant preference for the second and third pair of gills, and shows marked affinity for anterior hemibranches of each gill branch, and medial sections of each gill hemibranch. The results suggest that the larger volume of water flows and surface area of the second and third pair of gills might affect the distribution of M. sebastis, and the concentrated distribution of M. sebastis on the anterior medial section of gills would be related with the increasing chances of mating by niche restriction.