Weed management in lowland rice production is a major constraint leading to low yields. Studies were conducted during the wet cropping seasons of 2009 and 2010 at Rokupr Agricultural Research Centre (RARC) in the lowland ecology to assess the competitiveness of different rice varieties and to identify plant parameters associated with competitiveness. The experimental design was 6 x 2 factorial arranged in randomized complete block design (RCBD) in three replications. Six rice varieties and two systems (weed-free and in competition) were evaluated. The results showed varietal differences in their competitiveness against weeds. Average yield losses ranged from 13 to 67 percent in 2009 and 12 to 70 percent in 2010. With the exception of plant height, leaf area index (LAI) and tiller number correlated positively with competitiveness. The varieties NERICA L19, NERICA L20 and WAS 57-B-B-17-3-3-6-TGR20 an Inter-specific were found to be good competitors and high yielding, whilst Buttercup (the local variety) though competitive was low yielding. The varieties, NERICA L38 and ROK10 were the worse competitors but yielded similarly as NERICA L19 and NERICA L20 under weed-free plots. Therefore screening of rice varieties for competitiveness is important and could serve as a tool in breeding programmes to increase the competiveness of highly productive rice plant types without significantly affecting yields.