The method of transmission electron microscope was used to study the tissue distribution and formal characteristics of the microsporidian, Ameson sp. in the host Portunus trituberculatus. The results showed that the microsporidians were distributed mainly in the skeletal muscle, hemolymph, stomach and gut, no found in the heart, hepatopancreas, gonad and nervous tissues. Some meronts were found in the host skeletal muscle, connective tissue of the stomach and gut, indicating that the microsporidian was able to multiply in these tissues, especially in the skeletal muscle cells.A large number of microsporidians of different periods implied the tissue tropism of this microsporidian. The meronts and other multiplicative cells of the microsporidian were observed only in host cells, and the mature spores could exist within host cells and extracelluar matrix. A total of six existing forms of mature spores were found. The three within host cells:① The free spore without a membrane surrounding directly parasitized within the host cytoplasm. ② The spore was surrounded by a single membrane with the microvilli-like projections partly dissolved. This form was generally observed within an agranular cell (i.e. the professional phagocyte). ③ The spore was surrounded by lamellar membranous whorls and the microvilli-like projections was clearly visible. This form was found in nonprofessional phagocytes. The three within the host extracelluar matrix:① The spore was free without a membrane surrounding. The microvilli-like projections were clearly visible. ② Several spores were surrounded by a humoral encapsulation. The microvilli-like projections were clearly visible. ③ The spore was free without a membrane surrounding. The exospore and microvilli-like projections both disappeared. The present study sheds light on the tissue distribution of microsporidian and existing form of the spore, and provides important data for the further study on the migration of microsporidia in the crab.