The continuously-flooded rice production system in Chile has selected highly competitive aquatic weeds selection capable ofreducing paddy yields by 25 to 50%. Penoxsulam is a broad-spectrum triazolopyrimidine (ALS inhibitor) commercially introduced in Chile in 2006, where Alisma plantago aquatica hads already evolved resistance to sulfonylurea herbicides (SU). Several field trials with water seeded rice were conducted during 2003 and 2004 in Chile to determine efficacy, timing of application, spectrum of control, and control of SU-resistant A. plantago aquatica with the herbicide penoxsulam, either in single applications or in sequence with other herbicides. Penoxsulam was applied a) 12 days after seeding (DAS) into the water (IW), b) in postemergence after draining the water from field (ADW) at 35 DAS or c) IW followed by ADW applications of MCPA, cyhalofop, bentazon, or triclopyr. Penoxsulam was tested at 20, 30, and 40 g a.i. ha-1. Comparisons were made with recommended rates of other IW treatments (metsulfuron, bensulfuron and cyclosulfomuron) and sequential applications of molinate (IW) followed by ADW applications of MCPA, cyhalofop, bentazon, or triclopyr. One IW penoxsulam application was sufficient to achieve broad-spectrum control: Echinochloa spp. (100% control), A.plantago-aquatica (80 to 100%), Schoenoplectus mucronatus (50 to 80%), and Cyperus difformis (80 to 100%). Rice yields in penoxsulam-treated plots were 30 to 56% higher than in the untreated controls. Yields with penoxsulam IW (all rates) were similar (P>0.05) to those obtained using molinate (IW) followed by ADW applications of bentazon or MCPA. Although penoxsulam is an ALS inhibitor, it controlled A. plantago-aquatica resistant to metsulfuron and bensulfuron.