TWO SEPARATE FIELD EXPERIMENTS WERE CONDUCTED AT SHIRAZ AND KARAJ TO EVALUATE THE RESPONSES OF WINTER WHEAT (TRITICUM AESTIVUM L.) TO VARIOUS DENSITIES OF WILD BARLEY (H. SPONTANEUM. KOCH) AND NITROGEN (N) LEVELS. AT SHIRAZ AND WITH N, WHEAT PLANT HEIGHT AND TILLERS PLANT-1 WERE SIGNIFICANTLY DECREASED BY WILD BARLEY DENSITY OF 80 PLANTS M-2, WHEREAS WITHOUT N, ALL WEED DENSITIES SIGNIFICANTLY DECREASED WHEAT HEIGHT. WITH AND WITHOUT N, WILD BARLEY DENSITY OF 80 PLANTS M-2 SIGNIFICANTLY DECREASED WHEAT LAI. AT 390 KG HA-1, WILD BARLEY COULD PRODUCE HIGHER LAI THAN WHEAT. AT KARAJ AND WITHOUT N, WILD BARLEY DENSITIES OF 80 AND 160 PLANTS M-2 SIGNIFICANTLY DECREASED WHEAT PLANT HEIGHT, LAI AND TILLERS PLANT-1. WILD BARLEY HAD GREATER THE NUMBER OF TILLERS PLANT-1 AT DENSITIES OF 20 AND 40 PLANTS M-2. WITH N, WHEAT GRAIN YIELD WAS SIGNIFICANTLY DECREASED BY WILD BARLEY DENSITIES OF 80 AND 160 PLANTS M-2. THE RESULTS SUGGEST THAT WILD BARLEY HAD HIGHER COMPETITIVE ABILITY AT ALL N LEVELS AND ITS COMPETITION WITH WHEAT FOR NITROGEN WAS DIFFERENT IN TWO SITES. BECAUSE OF HIGHER LOSSES IN WHEAT GRAIN YIELD AND YIELD COMPONENTS DUE TO ALL WEED DENSITIES AND ALL N LEVELS AT SHIRAZ, WILD BARLEY COMPETITION AT SHIRAZ CONDITION MAY BE HIGHER THAN THAT OF KARAJ.