The aim of the study was to evaluate the yielding, yield components and the level of weed infestation of four cereal species which were grown with undersown serradella, or without undersown crop, in organic farms. A field experiment was conducted in the years 2011-2013, in a split-plot design, with four replications. The experiment involved two factors. The first factor included four species of cereals: winter rye, spelt wheat, spring barley, oats, while the second factor - the method of cereal cultivation: pure sowing, or sowing with undersown serradella. The studies showed that oats yielded the best, both in pure sowings and with undersown serradella, while spring barley - the worst. Undersown serradella favorably affected the yield of oats, reduced yield of barley, but did not affect significantly the grain yield of winter rye or spelt wheat. Yielding of serradella depended on the species of cereal as a cover crop. Its highest yields were obtained when it was undersown into winter rye, significantly lower - into barley and spelt wheat, while the lowest - into oats. Undersown serradella generally positively affected the number and weight of grains per ear, and per plant, as well as thousand grain weight. A species of cereal and the method of its cultivation had a significant effect on the abundance of fresh and dry weight of weeds. The highest weed infestation was recorded for pure cereal sowings. Undersown serradella reduced a weight of weeds on average by 48%, and their number by 45%. Mixed sowings with oats and spelt wheat were more competitive against weeds, than sowings with barley and rye.