One-factorial fields experiments were conducted in 1996 and 1997 in order to compare grain, fat yield and fatty acid composition of naked oat cv. Akt, grown in pure sowing and with serradella cv. Mazurska Biała (30 kg/ha). Grain fat content was determined according to the standard AO AC method, and fatty acid composition using a Varian 3400 gas chromatograph. All data were evaluated by analysis of variance and LSD (0,05) values were calculated. The yield of cv. Akt grown as a pure culture tended to give lower yields when compared to those obtained with undersown serradella. The presence of serradella increased slightly grain yield in two consecutive experimental years (1996 and 1997). Grain fat content of cultivar Act was higher then in covered oat. The sowing technique (pure and undersown serradella) had no apparent effect on the content of saturated, monounsaturated, and polyunsaturated fatty acids in oat grain. However, covered oat showed lower content of saturated and higher content of polyunsaturated fatty acids.