Abstract: This paper dealt with the sporogensis of Myxobolus rotundus Nemeczek, 1911 from Carassius auratus. Sporogensis of M. rotundus was observed by transmission electron microscopy. The earliest stage of the parasite was a primary cell which could give rise to generative cells by fission. Sporogensis was initiated by the envelopment of one generative cell with a enveloping cell. The enveloping cell did not divide anymore, whereas the large, generative cell divided to form a disporous pansporoblast containing 10 cells. Subsequently, the 10 cells began to differentiate and finally formed two 5 cell spore producing units. Each unit consisted of 2 capsulogenic cells and 1 binucleate sporoplasm, surrounded by 2 valvogenic cells.Each capsular primordium and its connecting external tubule formed a polar capsule which contained 7 helically coiled polar filament. The opening of the polar capsule was plugged by a stopper facing the discharge canal. Two valvogenic cells formed the spore valves. A desmosome like junction appeared in close contact the suture between two valvogenic cells. There were numerous microtubules at both sides of sutural ridge. The binucleated sporoplasm cell contained numerous sporoplasmosomes and spherical lipid inclusions in different size. Iodinophilous vacuoles, membrane bounded granules and membrane bounded vacuoles existed in the sporoplasm cell.In nearly mature spores, there was an unclear zones among sporoplasm cells. Several finger like extensions of sporoplasm extended to the zones.Other ultrastructural features were the same as those found in other myxosporeans, including absence of centrioles, the structure of mitochondria, distribution patterns of ribosomes and existence of glycogen particles, etc.