Abstract: Many researches reported that Stenotrophomonas maltophilia infected human in last twenty to thirty years,but we found it could make Ictalurus punctatus dead for the first time.Extracellular protease was one of the important pathogenic factors of Stenotrophomonas maltophilia.In order to investigate the character of extracellular protease and the optimal environment,the effects of protein modification reagents,tmperature,pH and metal iron on the S.maltophilia extracellular protease activity had been studied.The protein modification of extracellular protease of S.maltophilia was studied with PMSF,PCMB,N-AI,Ch-T,NBS and 2-ME as the modification reagents.The results indicated that the protease was not affected by N-AI,PMSF and PCMB,but Ch-T,NBS and 2-ME could reduce the activity significantly.It implied that Ser,Tyr and sulfhydryl residues were not activity dependent groups,while Met,Trp and disulfide linkage groups played an important role on the activity of extracellular protease.Meanwhile,we studied the effect of different environments on the protease activity.It showed that the optimal temperature was 20℃ and the optimal pH was 9.0.PMSF could not affect the activity but EDTA,Ca2+,Hg2+and Cu2+ could reduce the activity of protease greatly,however,Co2+ could enhance the activity.It demonstrated that the protease was a metal-protease.Meanwhile,the purified protease was injected to rats and Ictalurus punctatus intraperitoneally,which came out a huge death.The LD50 of extracellular protease to rats and Ictalurus punctatus were 4.33μg/g和3.49μg/g respectively.