Kudoa pericardialis n. sp. (Myxosporidea: Chloromyxidae) was described from the pericardial cavity of cultured yellowtails, Seriola quinqueradiata TEMMINCK et SCHLEGEL, in farms in Ajiro Bay, Shizuoka prefecture. Trophozoites were elliptical, oval or irregular and were free in the cavity or enveloped with membraneous tissues proliferated on the surface of the heart and the cavity wall. Spores were quadrilateral in polar view and were characterized by the four elongated polar capsules. The existence of the polar plate and the polar lid was demonstrated with the scanning electron microscope.