Abstract：Histopathological changes were observed in brain ,heart ,gill ,fin ,head kidney ,midkidney , premaxilla ,skin and viscus in juvenile grass carp Ctenopharyngodon idellus infected with protozoon Ichthyophthirius sp .The protozoon was found in various organs included gill filaments ,premaxilla , pectoral fins and dorsal fins ,and there was congestion in the brain ,and the auricle ,ventricule and arterial balls of the heart .There was congestion in the gill filaments with inflammatory cells infiltration ,the disappeared partly gill lamella structure ,and respiratory epithelial cells .The congestion in the head kidney , the fatty degeneration and hyalinization of epithelial cells of renal tubuls and the connecting tubule ,and enlargement of glomerula led to the disappearance of renal follicle ,with congestion in renal interstitium . The collagen fiber of dense layer in abdominal subcutaneous tissue showed part hyaline degeneration ,the congestion vein ,the intestinal mucous epithelial cell shed partly ,and the almost dissolved epithelial cells . The serious congestion was observed in the hepatopancreas ,and venous congestion appeared in the liver sinusoids and spleen .
