Abstract: This paper dealt with the structure of the mature spore of Myxobolus rotundus Nemeczek, 1911 and Myxobolus guanqiaoensis Wu Wang, 1997 under the optical and scanning electron microscopy. M. Rotundus was found in the body surface, head, gill, lip, and fin of Carassius auratus . It formed various inch cysts. M. guanqiaoensis was found in the liver of allogynogenetic silver crucian carp. It was found by histopathological observations that heavy infection of the spores could destroy the hepatic tissue.Under scanning electron microscopy, surface of the spore of M. rotundus was generally smooth. Its sutural line was straight and distinctive, running near the middle of the valves. Width of the sutural ridge was about 0.2 μm. Surface of the spore of M. guanqiaoensis was lightly shrunk. Its sutural line was S shaped and about 0.5 μm in width. The both valves of the spore were not bilateral symmetry. A furrow was observed in middle of sutural ridge. No striation pattern was found on surface of the spore, and no mucous envelope was around the spore in the two species.