The results of two-factorial field experiment conducted in 2002, 2004 and 2005 at the Research Station Mochełek near Bydgoszcz (53^o13’ N; 17^o51’ E) were applied to estimate mutual interactions between oat and yellow lupine. The factors were: sowing rate of oat (0, 140, 280, 420, 560 grains·m^-2) and lupine (0, 25, 50, 75, 100 seeds·m^-2). Combinations of levels of both factors gave 16 mixtures and four pure sowings of each of the species. Assessment of competition intensity was made based on indexes worked out by Rudnicki and Kotwica [2007]. Competition between oat and lupine in mixture stands is characterized by strong asymmetry. Oat was the dominant species and its competitive advantage over lupine resulted from a greater plant density. The competitive potential of a single yellow lupine plant towards oat turned out to be higher than that of a single oat plant towards lupine. Relations of the competitive strength of a single plant in mixtures were arranged according to the regularity that the stronger was competition between plants of species B and the weaker between plants of species A, the stronger competitors single plants of species A were for single plants of species B.

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