A kind of leeches, Trachelobdella sp was detected on the inside surface of the opercle of wild crucian carp, Carassius cuvieri (21.5 cm in length and 238.5 g in weight) at a reservoir for agriculture in ChungNam Province of Korea in March, 2002. Two or three round transparent spots on the opercle could be easily detected. The fish also showed severe anemia and dorsally darking in the gill lamellae. The parasites was identified as Trachelobdella sp. on the basis of morphological characteristics such as suckers, body segmentation and lateral diverticula. As the parasites fed on host blood after attaching on the inside surface of the opercle by means of the posterior sucker, they caused severe anemia and petechial haemorrhge on the gill of infested fish. Heavy infestation of the parasite resulted in dath of fish because of large amount of blood loss