This paper deals with histopathological comparison of six gill diseasescaused by parasites in grass carp. These diseases are Cryptobiasis, Trichodiniasis,Ichthyophthiriasis, Dactylogyriasis, Gyrodactyliasis and Sinergasiliasis. Our observations bear out, all parasitic gill diseases are able to cause the inflam-matory reaction, the gill hyperfunctionally secrete mucus and the eosinophilic andlymphocytic infiltration. However, there are some obvious differences in these diseases caused by various parasites each other. The seriously harmful diseases are degenerativeinflammation, such as Cryptobiasis, and the slightly harmfu1 diseases are proliferativeinflammation, such as Sinergasiliasis.