The preliminary studies to use dichlorvos (80%) for controlling the gill dactylogyriasis (Dactylogyrus vaginulatus Zhang et Niu, 1966) of silver carp,Hypophthalmichthys molitrix (Curier et Valenciennes) in the Jinjia and Houlou Reservoirs during 1991-1993 were reported. The method to sprinkle drugs for preventing and treating the reservoir fish diseases was discussed. The results showed that dichlorvos, 0. 08-0. 20 ppm, can thoroughly kill the trematode on gill of the silver carp. The histopathological studies indicated that the gill infected by trematode were with more mucus, injured very mechanically the gill filaments congested and bled. The results also showed: the resporatory epithelium swelled; hyperplasis of epithelial cell resulted in the fusion of adjacent lamellae to form the baculiform filaments. In serious condition,adjacent gill filaments further fused and formed a piece of epithelial cell plate.