In 2007-2009 the study was carried out and the aim was to evaluate the level of weed infestation of legume-cereals mixture cultivated in organic farming depending on natural fertilization. The study was performed at the Institute of Soil Science and Plant Cultivation - Agricultural Experimental Station Grabow (Mazowieckie Voivodeship). Eight mixtures of legume (pea, vetch) with cereals (barley, oat) in different share of components (50:50% and 75:25%) were tested. The qualitative and quantitative analysis of weed infestation were done. The weed communities were compared using Shannon's diversity index and Simpson's dominance index. The study showed that species diversity, number of weeds and weight of weeds were the largest in mixture of barley (50%) with pea (50%). The analysis using Shannon diversity index revealed the biggest weed flora in this mixture. The smallest number of weeds was in oat-vetch mixture. Simpson's dominance index showed a domination of one species in barley-vetch mixture.