In order to research the possible mechanism that the duration of molting cycle of the juvenile prawn Macrobrachium nipponense was shortened by KK-42,the cuticle structure of carapace was investigated by the histological method and the N-acetyl-β-D-glucosaminidase(NAGase)activity from hepatopancreas was quantitatively analyzed using ultraviolet absorption method before and after KK-42 treatment.The prawns with body length of (3.3±0.5)cm were randomly divided into two groups:KK-42 treatment group and control one.The prawns were soaked for 1 min in KK-42 solution at a concentration of 1.95×10^-4 mol/L(KK-42 treatment group)or 0 mol/L(control group),respectively.Afterwards,the carapaces derived from late premolt(D₃)prawns were chosen to be used for histological observation and thickness measurement of layered exoskeleton,meanwhile,the hepatopancreas NAGase activity in stage D₃ M.nipponense was quantitatively analyzed.The results showed that the exocuticle thickness of carapace significantly increased by 42.83% compared with the corresponding control on 1^st day after KK-42 treatment,and the endocuticle thickness as well as the whole exoskeleton thickness significantly increased by 70.72%(P<0.01),77.27%(P<0.01)and 38.69%(P<0.01),52.23%(P<0.01),respectively,compared to the corresponding control on the 3^rd day and the 9^th day after KK-42 treatment.A 31.69% rise in the height of epithelial cells was obtained on the 3^rd day after KK-42 treatment compared with the corresponding control.The NAGase activity significantly increased by 42.53%(P<0.01),34.28%(P<0.01)和18.36%(P<0.05),respectively,at 6 h,12 h and 24 h after KK-42 administration compared to the corresponding control.The results reveal that KK-42 treatment can significantly increase the cuticle thickness of carapace,and promote NAGase activity from hepatopancreas.