Red rice (Oryza sativa L. var. sylvatica) is an annual weed which has been reported to cause severe infestations throughout Europe. The objective of this study, carried out in the 1997-1998 period as to determine the competitive relationship between 4 red rice ensities and 2 long-cycle rice varieties, continuously grown in flooded conditions. The red rice densities were 0, 10, 20 and 40 plants m-2. The cultivated varieties were Thaibonnet, which is characterised by a short height and long grain and Arborio, which is instead characterised by a tall height and medium grain. Both crop varieties were broadcast planted in a flooded field at 600 seeds m-2. The experimental design was a randomized block, with 50 m2 plots and 4 replications. No significant differences occurred over the years for all the parameters considered to evaluate the weed crop interactions. The red rice densities of 10, 20 and 40 plants m-2 reduced the rough grain yield of commercial rice in Thaibonnet and Arborio by 26.5, 32.8, 52.0% and 29.7, 41.3, 60.5%, respectively. The leaf area of red rice was 2.5 and 3.2% higher than in Thaibonnet and Arborio, respectively and was not influenced by weed density. The red rice density did not remarkably affect the crop height either. In all the competitive conditions, the red rice was 36% taller than Thaibonnet and as tall as Arborio, since the beginning of the stem elongation. Red rice at 10, 20 and 40 plants m-2 reduced straw dry weight of cultivated rice 24.5, 35.5 and 42.2% in Arborio and 72.3, 83.4, and 87.1% in Thaibonnet, respectively. The red rice grains m-2 counted on the soil surface immediately after crop harvesting were 2056, 4833, and 7081 in Thaibonnet and 2743, 4325, and 9618 in Arborio at 10, 20, and 40 red rice plants m-2, respectively. The results of this research pointed out that red rice infestations, already from the density of 10 plants m-2 resulted in a severe yield reduction of both rice varieties and in a great risk of weed seed dissemination.