To avoid the self-agglutination of Staphylococcus aureus sensitized with rabbit antibody in the absence of antigen, we determined the optimum concentration of rabbit antibody for sensitization. It was analyzed by using three different kinds of S. aureus strains at various concentraions of antibody. The optimized coagglutination test using the S. aureus sensitized with rabbit antibody was applied to the diagnosis of edwardsiellosis in field and in laboratory. The presence of E. tarda as low as $10;{mu}g/ml$ was detected by this method. Moreover, it showed good coagglutination results against several different forms of antigens such as FKC, EDTA or heat extracted antigen of E. tarda. E. tarda strains, isolated from the flounders suffering from edwardsiellosis in fields, showed some cross-reactions to the E. tarda 219 analyzed by both agglutination and coagglutination test with rabbit anti-E, tarda 219 antibody. The degree of cross-reactions analyzed was enough to apply the coagglutination test for the diagnosis of edwardsiellosis in field. Thus, even 1,000 fold diluted tissue homogenate of infected flounder naturally contained enough E. tarda as an antigen to show good coagglutination with S. aueus sensitized with rabbit anti-E, tarda 219 antibody. The successful application of this method to the homogenate and heat extract of tissues from naturally or artificially infected flounder or tilapia preyed that coagglutination test was a simple and rapid reliable dignostic technique suitable for using in laboratory and field without any special equipments.