Field experiments were conducted to evaluate the bioefficacy of phosmet 50 WP against C. medinalis on rice. The first experiment was conducted during October 2017 to February 2018 using the variety BPT 1024 at Sivapuri village (79º42’48.3”E) near Chidambaram and the second experiment was conducted during October 2018 to February 2019 using the variety BPT 1024 at Nanjamahathuvalkai village (11º23’53.49”N and 79º41’43.29”E) near Chidambaram. Five doses of phosmet 50 WP @ 400, 500, 600, 700, 1000 g a.i/ha and the standard check Cartap hydrochloride 50% SP @ 500 g a.i/ha was tested against leaffolder infesting rice. The leaffolder incidence was reduced to an extent of 64.11- 88.24 per cent in phosmet 50 WP during first season. The effectiveness of phosmet 50 WP against leaffolder during second season was reduced to an extent of 58.70- 86.50 per cent. Phosmet 50 WP @ 600 g a.i ha^-1 was highly effective in reducing the per cent damage of leaffolder as the per cent reduction over control on 7 and 14 DAT respectively.