Infestation of juvenile gnathiids, Gnathia sp., on the branchial chamber wall of two species of stingrays, Dasyatis akajei and D. matsubarai, caused proliferative inflammation of dermal tissue, and a heavy disruption of epithelia and smooth muscles. In addition to congestion of hemolytic blood cells, a prominent aggregation of heterophils, macrophages engaged in active phagocytosis, and plasma cells was also seen. It is of interest to note that evidence of a remarkable propagation of bacilli was encountered on the surface of the gnathiids and on the injured mucosal coat of the host fish. Besides a lot of blood sucking by gnathiids, bacterial infection might be one of the causes of host fish death or debility.