The paper presents the results of floristic composition and weed biomass in organic alfalfa crop in different years of alfalfa growing. The experiment was conducted in experimental plots in Futog, during the vegetation period of 2015. The presence of 60 weed species was noted, 49 of all weeds were noticed in the third, 48 in the fourth and 34 in the fifth year of organic alfalfa growing. Quantitative and qualitative structure of weed community depended on the age of alfalfa crop. By the analysis of cover values and the degree of the presence of weed species, it can be seen that species: Rumex obtusifolius L., Taraxacum officinale Web., Cynodon dactylon Pers. and Artemisia vulgaris L. had the greatest significance on weed infestation of alfalfa. The largest number of weed species belonged to the families Poaceae and Asteraceae. The biological spectrum of weeds species was defined by the terophyta dominance, whose number was reduced due to the age of the crop, while the share of hemicryptophytes and geophytes was more or less constant. Although we found a considerable level of infestation of alfalfa, alfalfa biomass reached acceptable values that were significantly higher than the weed biomass, which was statistically highly significant.