Fauna and seasonal variation of Myxosporidian and histopathology in common carp were reported in this paper. 612 specimens of common carp were examined in 1987-1988. 41species of Myxosporidian were found. Among them, the majority of species were Myxobolus (23 species). The remainings were Thelohanellus (7 species), Myxidium (4 species), Myxosoma (4 species), Sphaerospora (2 species) and Henneguya (1 species). 17 species of these parasites were found in the gill. Thelohanellus hovorkae and Myxobolus koi were dominant. Strongly infecting parasites were Sphaerospora molnari, Myxobolus koi, Myxobolus artus, Myxobolus dispar, Myxidium leiberkaehni, Myxidium cuneiforine, Thelohanellus hovorkae and Thelohanellus nikolskii. In the infection of Myxobolus koi in the common carp, a two-peaked seasonlity (in spring and winter) can be observed. There is a strong negative correlation between size classes of common carp and infection percentage of Myxobolus koi. Infection peaks of Myxobolus artus were found in spring and autumn (appropriate temperature 17-26). A one-peaked seasonality of Thelohanellus hovorkae can be examined in summer. There is a significant positive correlation between infection percentage of thelohanellus hovorkae and temperature chages. Histopathological responses of host tissues caused by three species of Myxobolus were described. Myxobolus koi causes hyperplasia, withering, or inflammation of the gill filament, and may seriously interferewith gill function. Myxobolus dispar dissolves cartilaginous tissue, and erodes connective tissue of the gill arch, and hinders opening or closing of the gill filaments. Swelling of blood cavity of the gill lamellae caused by Myxobolus artus affects gill respiratory function. Myxobolus artus causes seiousely blood-poor in the gill, and degeneration and dissolvation of the muscle, and produces encysted abscess.