All of the crucian carp, Carassius auratus and 50% of common carp. Cyprinus carpio examined in this study were infected with the leeches, Limnotrachelobdella sinensis. Especially, the infection of C. carpio with L. sinensis was the first report in Korea. The gill of C. auratus showed increased hydrophic degeneration of epithelial cell in the filament, blood congestion, hyperplasia of epithelial cell in the filament and lamellae. In the SEM observation, gill filament was transformed to the cylinder form by the lamellae fusion. The lamellae surface showed degeneration, fragmentation of microridges. The extracellular cartilaginous matrix of the filaments was exposed by the collapse of epidermal layer. In the 18S rRNA analysis of L. sinensis, the relationships among these groups are not clear and not concord with their morphological classification.