Abstract: Farnesoic acid O-methyl transferase(FA-O-MeT)catalyzes the methylation of farnesoic acid(FA)to methyl farnesoate(MF).Here we analyzed the activity of FA-O-MeT in mandibular organs(MOs)of Procambarus clarkii by the radiochemical assay,and also detected the regulation of eyestalk factor on the activity of FA-O-MeT.This work showed that there was no significant difference between the right and the left MO in the FA-O-MeT activity(P>0.05); During the different ovary developmental stages,the enzyme activity was also at different levels,and there was a peak at the secondary vitellogenesis stage,to 61.75 pmol/(p·h);Eyestalk factor removal increased FA-O-MeT activity,and the peak appeared on the 5th day.Furthermore,incubation MOs in the DMEM with the SG extracts significantly decreased the FA-O-MeT activity(P<0.01).These results indicated that the activity of FA-O-MeT is closely related to ovarian development and is negatively regulated by X-organ sinus gland complex in the eyestalk.