Summary:The competitive interactions between Avena sterilis ssp. ludoviciana (Dur.) Nyman and winter barley have been studied, taking into consideration the densities of both species. As the density of A. sterilis increased, barley yield decreased exponentially. A 10% reduction in yield was found with wild oat densities ranging from 20–80 panicles m–2, and yield losses reached 50%, with densities of >300 panicles m–2, Barley grain yield was reduced by wild oats through a reduction in the number of fertile tillers. Climatic conditions during the growing seasons affected the response of barley to wild oat competition. In general, barley yields were relatively unaffected by seeding rates, with similar responses observed in the presence and in the absence of wild oat infestations. However, the highest yield losses were obtained with the lowest seeding rate (100 kg ha–1). Furthermore, low barley densities allowed the wild oat plants to produce more seeds, increasing the potential infestation during the following season.