In the southern sea of Korea, the culturing Rock bream, Oplegnathus fasciatus, ranging from 16.3 to 20.1cm in body length were sampled to examine how serious parasitic infection is. As a result, they were infected with 10 to 31 individuals of Monogenea, Benedenia sp. The parasitic sites of fish body was trunk (81.8%), caudal peduncle (16.5%) and the other portion (1.6%), and where ulceration and bleeding were observed with a large amount of muscus. When the treatment with formalin and freshwater were performed to get rid of the parasite, they were exterminated after 8 and 20 minutes in 100% and 70% freshwater, respectively and were killed after 10, 15 and 20 mimutes in the seawater added with formalin to be 250 ppm, 200 ppm and 150 ppm, 200 ppm and 150 ppm, respectively. During the treatment for parasite extermination, the fish showing a serious illness were dead in the 100% freshwater, whereas toxicity caused by formalin treatment was not detected in this study. The former is probably due to failure in the osmotic regulation of Rock bream.