Abstract: The resistance of enzymolytic chitooligosaccharides from housefly larvae against Phytophthora parasitica var.nicotianae(P.parasitica) and the mechanism of that were studied preliminarily.The results showed that chitooligosaccharides greatly affected the morphology of mycelia and sporiparous ability of P.parasitica,which resulted in hyphal distortion,stunt,and swollen branches,at the same time,inhibited zoosporangia production.The tobacco seedling treated with chitooligosaccharides prior to P.parasitica inoculation obtains induced resistance against tobacco black shank,and the activities of isoenzymes peroxidase(POD) and polyphenoloxidase(PPO) in tobacco plant increase significantly,pathogen inoculation after chitooligosaccharides spraying causes that the isoenzymes in tobacco plants had more enzyme bands and deep in colour.Therefore,it is considered that the resistance of enzymolytic chitooligosaccharides from housefly larvae against black shank is related to these two isoenzymes.