Abstract: The pathological changes of myxosporadiosis naturally infected by Myxobolus rotundus in crucian carp (Carassius auratus auratus)were studied by conventional paraffin section and hematoxylin-eosin staining. Results show that many milky white cysts at different sizes form on the head, gill and skin of the infected crucian carp; the cysts have a typical 3-layer structure: connective tissue, ectoplasm with double membranes and endoplasm with the spores or trophozoites at different developmental stages. Pathological changes are mainly shown as follows: swollen and proliferated branchial filament in the gill; vacuolation of sinusoid-centric cells in the liver; epithelium necrosis in renal tubules and atrophia in renal glomerulus; necrosis of intestine mucous layer and swollen degeneration of intestine muscle layer; abnormal increase of hemosiderin in spleen.