A field experiment was conducted during wet season of 2006 and 2007 at Ghaghraghat, Uttar Pradesh to find out the comparative efficacy of different formulations of penoxsulum on weeds and on yield in transplanted rice in lowlands. The experiment was laid out in randomized block design with 7 treatments replicated 4 times. The major weed flora recorded in the experimental site was Echinochola crus-galli, Echinochola colona, Commelin benghalensis, Caesulia axillaris, Cynotis axillaries, Ammania baccifera, Cyperus spp., Cynodon dactylon. Weedy check till maturity reduced the grain yield of rice to the tune of 43.65%. The lowest density of weeds was recorded with two hand weeding at 20 and 40 days after transplanting, however, application of penoxsulum @ 22.5 g a.i. ha^-1 at 8-12 days after transplanting resulted the lowest dry weight of weeds and higher weed control efficiency at 30, 60 and 90 days after transplanting. The grain and straw yield were highest (29.53 tons. ha-1 and 36.71 tons. ha^-1), respectively with penoxsulum @22.5g a.i. ha^-1 8-10 days after transplanting. This treatment also resulted the lowest weed index (18.88%) followed by penoxulam @25g a.i. ha^-1 @ 0-5 days after transplanting.