Abstract: The mechanism of chitooligosaccharides from housefly larvae against tobacco black shank were studied preliminarily.The results showed that chitooligosaccharide-treated tobacco seedlings obtained induced resistance against tobacco black shank,after inoculating Phytophthora parasitica var.nicotiana.The induced resistance improved with the increase of chitooligosaccharides concentration within the range of 0.1-1.5 mg/mL,its relative control effect was 92.21% at the concentration of 1.0 mg/mL,which was not significantly different from that of 1.5 mg/mL.At the same time,the activities of phenylalanine ammonialyase,peroxidase and polyphenoloxidase in tobacco leaves increased in different degrees and peaked on the 8th day,5th day and 3rd day after treating,which were 94.62%,160.44% and 89.76% higher than those in control leaves,respectively.