Lowland ecotype Cyperus rotundus L. has been reported infesting irrigated lowland ricefields in the Philippines. Its effects on growth and yield of rice under flooded conditions are unknown. Two experimental runs were conducted in a screenhouse to determine the growth of lowland ecotype C. rotundus with transplanted rice and wet direct-seeded rice at a sowing density of 0, 22, 44, 66, and 88 tubers m -2 ; and its effect on growth and yield of rice. Except for height, growth variables of lowland ecotype C. rotundus were increased with the increase in its initial tuber densities. Compared with 22 tubers m -2 , its number of off-shoots and tubers, and shoot and root biomass at 44 to 88 tubers m -2 increased by up to 3x. Growth variables of rice were reduced by the weed. Grain yield of transplanted rice was reduced by 14 to 38%; wet direct-seeded rice by 11 to 28%. Results suggest that lowland ecotype C. rotundus can grow well and reduce growth and yield of rice under flooded conditions. It also imply the need of developing a holistic weed control strategy against the weed.