Field studies were conducted in northern Greece, in 1997 and 1998, to investigate the effect of nitrogen fertilization and red rice density on interference between red rice and two rice cultivars (Thaibonnet, Ariette). Interference between rice and red rice began 3 wk after rice emergence, but was not affected by the increasing nitrogen rate from 100 to 150 kg N ha–1. Dry weight of both rice cultivars was proportionally reduced with increasing red rice interference duration and density, but dry weight of Thaibonnet was reduced more than that of Ariette. At harvest, grain yield of Thaibonnet was reduced by 58% because of the occurrence of 40 red rice plants m–2, whereas that of Ariette was reduced by 46%. Red rice interference affected panicle number more than 1,000 grain weight in both rice cultivars. The reduction of all yield components was greater in Thaibonnet than in Ariette. Dry weight and stem or panicle number of red rice plants grown with either of the two rice cultivars increased with increasing red rice density and were greater most of the time when grown with Thaibonnet than with Ariette. Ten weeks after rice emergence, red rice plants were 14 and 35 cm taller than the Ariette and Thaibonnet plants, respectively. Shattering of red rice plants ranged from 63 to 79% and was greater grown with Thaibonnet than with Ariette, but it was not affected by nitrogen fertilization and red rice density.