Field studies were carried out in 1997 and 1998 in northwest Italy with the aim of determining the effects of weedy rice competition on rice growth dynamics and yield. ‘Arborio’ (medium-high tall size, high early vigour) and ‘Thaibonnet’ (semi-dwarf size, low early vigour) rice cultivars were grown in competition with weedy rice at four densities (0, 10, 20, and 40 plants m-2). For both cultivars the plant height, the aboveground biomass and the number of tillers were determined at different intervals. With some exceptions, all the measured rice growth parameters were not remarkably affected by weedy rice interference. Weedy rice plants grown in competition with ‘Arborio’ were generally taller and produced more biomass. Weedy rice competition resulted in up to 50% (in ‘Thaibonnet’) and 48% (in ‘Arborio’) yield losses. The yield losses observed in both varieties were primarily related to the reduction of yield components, and of panicle density and number of filled grains per panicle, in particular.