Abstract: Myxobolus artus Achmerov, 1960, is a pathogen which causes an instestinal disease in the fry of grass carp (Ctenopharyngoden idellus). It mainly inhabits the lomins proprina of fore-gut of the fish. The whole developmental processes of this parasite including mononuclear trophozoite, nuclear division, multinuclear trophozoite, monosporont or disporont of plasm division, polar capsule shaping, valvular wall forming and cyst appearance were observed in host's tissue under electron microscope at the temperature of 27—30℃. In fish tissue, it took about 12—15 days to attain maturity. Observation with transmission electron microscope showed that mature spores have rich rough endoplasmic reticulum, glycogen and have an irregular triangleshaped nucleus and two black unidentified cycloid-bodies near each polar capsule. The polar filament of polar capsule has 4—5 coils, Scanning microscopy shows that the sutural ridge of spores is rather protuberant and each valve has an opening for polar filament.