Abstract: Turbot,Scophthalmus maximus(L.) is a very important commercial species in Europe, from where it was introduced into China in 1992. With the success in artificial breeding from 1998, the commercial culture has spread rapidly along the coast of North China. Recently, yields have reached 20,000MT, valued at 200 million RMB. From 2003, a novel disease in turbot associated with Mesanophrys carcini infection was identified in China, causing high mortality and economic losses. This paper first reports the histopathology of this disease, aiming at supplying theoretical foundation for the prevention and therapy. In the case of infected larvae, ciliates could be found infin, skin, gills, eye and internal organs such as brain, heart, kidney, spleen, pancreas, liver and the alimentary tract, which indicated the systemic infection to the larvae. In infected large turbot, the ciliates were predominantlyfound in the skin, fin and gills. And in the severe cases, they could invade the brain, liver, kidney and heart with little numbers. Histopathological studies revealed that various pathological changes to the tissues associated with Mesanophrys carcini. The affected larvae and large turbot showed the consistent histopathological changes, including hyperplasia of the branchial epithelium, meningitis, liquefaction of the brain, necrosis of haemopoietic tissue in the kidney and spleen, dilation of central vein and sinusoids in the liver, coagulative necrosis of muscle fibers, congestion and oedema of choroid in the eye and proliferation of mononuclear macrophages. Mesanophrys carcini caused damage to the gills, resulting in the asphyxia of the diseased turbot. This is probably the major causes of death in affectedfish. And organsfailure as a result of necrosis associated with presence of Mesanophrys carcini is also responsible for the death of affected fish. In addition, we discuss possible route of entry into the host, considering that injured skin and gills are the main route for the Mesanophrys carcini infection. The study tells us that keeping pond cleaning, avoiding scratching the fish surface are very necessaryfor the prevention of the Mesanophrys carcini infection; the therapy should include not only external treatment but also the internal.