The duration of the 6th instar of the tobacco cutworm, Spodoptera litu ra, was prolonged　by the topical application of methoprene at a dosage of 0.25μg/μl. The maximum duration, 225.5±2.0 hrs, was obtained when methoprene was applied at 37.41(31.48～41.68) hrs after the 5th moulting. The application of methoprene at and after 97 hours after the moulting gave no effect on the prolongation of larval duration. Subsequent increment of pupal weight was also induced. The maximum pupal weight in female, 0.6015±0.0134g, was obtained by the application of methoprene at 43.31(21.35～53.35) hrs after the moul ting, and that in male was 0.5457±0.0099g at 41.34(14.75～52.91) hrs.