Summary.The object of the work described in this paper was to obtain information on the relationship between the effects of crop competition and barban treatment on oats growing in another cereal. The work was carried out on barley cross‐drilled with cultivated oats. Variation in crop competition was obtained by using different barley seed‐rates and nitrogen levels. Increasing the seedrate of barley without applying barban resulted in a reduction of the initial oat population. This reduction applied to weight of individual shoots, number of shoots and panicles per unit area and number of spikelets per panicle. The application of barban at selective rates gave an independent reduction of oats of the order of 80%. This value proved almost constant at each seedrate of barley. Crop competition and barban accordingly seem to art independently and their effects appear to be simply additive. The barley fresh weights per unit area increased substantially when oat competition was reduced by barban treatment. The highest increase occurred on the plots with the lowest barley seedrate in spite of the fact that these plots still appeared infested with oats at harvest time. Nitrogen application stimulated the growth of barley and oats to a similar degree and did not in consequence lead to a selective suppression of oats.Etude de l'influence de la densité du Semis, du taux de fertilisation azotée et du traitement au barbane, sur la concurrence entre une Orge et une