Cyperus rotundus L. was reported as dominant weed in upland rice areas and a minor problem in the rainfed lowland. However, it has evolved as second most dominant weed in rainfed and irrigated lowland rice fields (rice-vegetable system) in three villages of San Jose City, Nueva Ecija. Its occurrence in the monoculture rice production system has never been explored especially in irrigated lowland rice fields of Nueva Ecija. Field surveys and interviews were conducted in 2005 - 2006 to determine the prevalence of lowland ecotype C. rotundus and how farmers manage it in irrigated lowland rice fields of Aliaga, Nueva Ecija. In 2004, lowland rice fields of Nueva Ecija where the rice-rice cropping system was commonly established were surveyed. Aliaga was surveyed to calculate the incidence and map the distribution of the weed. To check whether C. rotundus was a problem, 40 randomly selected rice farmers were interviewed using a guided questionnaire. Results of the survey revealed that 9 of 26 villages in Aliaga had 1 - 10% infestation of C. rotundus; 3 had 11 - 20%; and 5 had more than 20%. Infestation was high in areas with low elevations. Even taller than cultivated rice, C. rotundus was the most commonly encountered and dominant weed species and had already existed for 5-20 years in the field. Herbicide application (MCPA at a rate of 1-1.5 L ha-1) was the most commonly used weed management practice against the weed.