The herbicidal activity and properties of a diphenylpyrrolidinone, MT-141 [4-ethyl-3-(3-fluorophenyl)-1-(3-trifluoromethylphenyl)pyrrolidin-2-one], were examined. MT-141 controlled barnyardgrass (Echinochloa crus-galli), johnsongrass (Sorghum halepense), green foxtail (Setaria viridis), large crabgrass (Digitaria sanguinalis), fall panicum (Panicum dichotomiflorum), goosegrass (Eleusine indica), and broadleaf signalgrass (Brachiaria platyphylla) at 300 g a.i./ha when applied pre-emergence (PRE), and provided greater than 90% control of these weed species at 500 g a.i./ha when applied post-emergence (POST). MT-141 was less effective against broad-leaved plants such as velvetleaf (Abutilon theophrasti) and ivyleaf morningglory (Ipomoea hederacea), but two other broad-leaved plants, hemp sesbania (Sesbania exaltata) and prickly sida (Sida spinosa), were slightly susceptible to MT-141. MT-141 applied PRE at 500 g a.i./ha did not injure cotton. The most significant herbicidal symptom for this compound was bleaching. Residual activity of MT-141 applied PRE to barnyardgrass and johnsongrass lasted at least 5 weeks. Planting depth or soil type did not affect the herbicidal activity of MT-141 at 300 g a.i./ha. MT-141 applied PRE increased the herbicidal activity of glyphosate against hemp sesbania and morningglory without injuring glyphosate-resistant cotton. Also several surfactants increased the herbicidal efficacy of this compound on POST application. MT-141 seems to be an effective herbicidal compound for controlling graminaceous weeds when applied PRE in cotton production. © Pesticide Science Society of Japan