The bacterial septicemia is initially found in crucian carp at ChongmingCounty, Shanghai, October 1986. And it spread rapidly to 20 provinces. This disease makesvery high epidermic and mortality on cultured fish, Its glaring symptoms are the hemolysisand a great quantity of ascites in abdominal cavity. Many species of freshwater fishes such ascrucian carp, blunt snout bream, silver carp, big head carp, common carp and rice field eelwere murdered seriously and caused enormous losses to fishermen. At that time nobodymade a clear idea to the pathogeny and pathology of this disease, and now it has beenknown. Their important pathogens are bacteria of Aeromonas hydrophila. The researchcourse of this disease listed briefly in literature of the present paper. In this paper, thesuper-microhistopathology of the bacterial septicemia and that of crucian carp, etc, will bediscussed comparatively. Their general pathological characters of diseased fish are extensivehemolysis and hemorrhage in whole body, all organs were disintegrate, particularly thesubstantive organs. Initial diseased change is the membraneous stuctures to be damaged inliver, spleen and kidney. These symptom may be perceived under electromicroscope but cannot perceived under the lightmicroscope. Under lightmicroscope, much filiform matter dyedby hematoxylin are seen. Under electromicroscope, the filiform matter is rough endoplasmicreticulum and concentrated into sheaves. This is a compensated action. The pathologicalchange degree of crucian carp was slighter than other fishes at same time that owing to itsmore vitality. According to the pathogen and pathological changes, a new compoundmedicine "YUTAI 8" was developed for this disease, and 3 steps of therapeutic method and16 points for attention were raised. 2000 ha of cultured ponds were treated and the cure ratewas over 95%. And using "YUTAl 8" for prevention in 300 ha of cultured ponds, the effectwas good too. In meantime, 10 multiple preventive measures were raised for building upresistance to disease and restraining the pathogens on basis practices.