By using unlabeled enzyme-immunoassay method to examine the gill disease cau-sed by the bacteria Myxococcus pisicola good results were obtained. The experimentsmanifest that the bacteria from different strains gave cross reaction to specific anti-serum, and the enzyme-immunoassay shows positive reaction. A brown color appeared,and the curved slender bacteria could easily be observed under the microscope. In thecross set, including the nomal rabbit serum, PBS-T, the absorbed serum, the Cg25strains or Cg27 strains were used to substitute the bacteria for checking. The results all gave negative reaction, and no brown color appeared. Evident differences could bedistinguished under microscope. If the mucus is taken from the gills of the healthygrass carp for checking, it appears light yellow in color, only the red blood corpusclesgiving dark brown color. This method of examination has certain practical uses for the survey of prevailingmucous bacteria disease and for the dignosis on other fish diseases. It may be also usefulin localizing pathogen of virulent diseases, in tracing the source and the path of infec-tion, and in studying bacterial and parasitological diseases of fishes.