Abstract: Regulations of cultural conditions on the exotoxin expression of Vibrio mimicus HX- 4 from Eriocheir sinensis have been studied in vitro. Through measuring hemolytic and proteolytic activities of crude exotoxin in different cultural conditions. The results demonstrated that different media, NaCl percent, initial pH, soluble oxygen, cultural temperature and time all effected on the exotoxin production. The optimal conditions of the exotoxin expression are that the HX- 4 strain is inoculated in 0. 5% NaC-l BHI with init ial pH 8. 0 at 28 e for 36h in shaking cultivation or 48h in stat ionary cultivation. The pathogenic experiments of the crude exotoxin to mice and Eriocheir sinensis also have been finished. The results showed that the crude exotoxin is pathogenic to both of experimental animals, the higher the hemolytic titer of crude exotoxin is, the higher its lethal rate to experimental animals is. The crude solut ion of exotoxin with protease and no hemolysin has pathogenicity, but no lethality to experimental animals.