Methyl farnesoate(MF)is an important hormone controlling crustacean growth.Our previous study has demonstrated that the treatment of juvenile hormone antagonist KK-42 can accelerate growth of Litopenaeus vannamei.In order to research the possible molecular mechanism of KK-42 growthpromoting effect onL. vannamei, a part of mRNA sequence of 3-hydroxy-3-methylglutaryl-coenzyme A reductase(HMGR),a key enzyme of MF precursor production,was cloned and the spatial and temporal expressions of HMGR and the effect of KK-42 on the expressions were assayed.The shrimps(L. vannamei),3.5-5.0 cm long,were soaked for 1 min in KK-42 solution at a concentration of 1.95×10^-4 mol/L or in the solution without KK-42,and then cultured in normal way.The HMGR mRNA levels derived from tissues were assayed using real-time PCR and hemolymph MF titer was determined by high-performance liquid chromatography(HLPC).Results showed that the highest level of HMGR transcript occurred in mandibular organ(MO),then in eyestalk and the lowest in muscle and hepatopancreas.The expression pattern of HMGR derived from eyestalk and MO tissues varied in control group during the experiment,of which no statistical difference in the mRNA level was observed in the eyestalk,but the mRNA content rose significantly on day 1 and day 7 compared to day 0 in the MO; the MF titer gradually increased during the experiment and was observed.Administration of KK-42 resulted in important depression on HMGR expression,and the change trend of the downregulated mRNA levels,however,differed in the two tissues mentioned.For eyestalk,the HMGR mRNA levels at all time points had no statistical difference,but those fluctuated and even reduced by 80.2%(P＜0.01)and 90.7%(P＜0.01)on day 1 and day 7 after KK-42 treatment,respectively,in MO.Meanwhile,the amount of hemolymph MF fell in KK-42treated shrimps compared to that in control ones during the experiment.Our present study demonstrates that treatment of KK-42 significantly inhibits HMGR expression in different tissues especially in MO,and decreases the hemolymph MF titer,which is likely one of mechanisms of KK-42 growthpromoting effect on L. vannamei.