Light and electron microscopical characteristics of Perkinsus sp. parasitizing in Manila clam, Ruditapes philippinarum, in Korea were investigated. Trophozoite within the tissue was spherical or ovoid and ranged $2.5-10.5mu m$ $(mean = 6.2mu m)$ in diameter. Trophozoite had a nucleus with a prominent nucleolus and a large cytoplasmic vacuole within the cytoplasm. Single trophozoite was phagocytozed by host hemocyte and cluster cells were encapusulated by hemocytes aggregation within the host tissues. Hypnospores incubated in thioglycollate medium (FTM) for 1 to 15 days were also spherical or ovoid and ranged $10-132mu m$ $(meanpm S.D.;:;44.25pm 7.91mu m)$ in diameter. Zoospores were spherical or ovoid, had a nucleus and two flagella. Zoospores contained apical complex, which consisted of conoid, subpellicular microtubules, rhoptries and rectilinear micronemes.