Abstract: An indirect haemagglutination test was empolyed to detect circulating antibodies in the sera of Carassius auratus Linnaeus infected naturally with Wyxobolus rotundus Nemeczek. The circulating antibodies were detected in naturally infected C.auratus. But statistical correlation between infection intensities and antibody levels was found. Polyantibodies were prepared by injecting New Zealand white rabbit with soluble antigen of M. rotundus. It was suggested that same antigens among different life stages of M .rotundus, and the genus-specific antigens be presented by using enzyme-linked immunoassay (ELISA). By using the indirect fluorescent antibody technique (IFAn, fluorescence was found densely concentrated in the rear point and periphery of spores of M. rotundus. However, the two polar capsules were obviously unstained in the fluorescent background. The anti-M. rotundus antiserum could react with M. guanqiaoensis Wu et Wang by IFAT, fluorescence was observed in sporoplasm and the anterior end of the sppre wall. At the same time, polar filaments were fainily fluorescent. Trophozoites of the two parasites could react with anti-M. rotundus antiserum. Fluorescence was scattered on the whole parasites.