Based on the highest specific combining ability (SCA) of lines in S5 generation, from nine original synthetic populations of alfalfa, generations Syn-1, Syn-2 and Syn-3 were obtained. Inbred lines were crossed wih Medicago falcata, which possessed high fertility genes and genes for few thermal requirements for pod and seed settlement. Synthetic populations were exactly analyzed for seed yield, fresh matter yield and a series of other characters connected with them. Moreover, genotypic response of synthetic populations to soil climatic conditions of Poland was defined. Based on the results obtained from the experiment, three synthetic populations, i.e. no. 4, 5 and 9, with the highest seed and fresh matter yield were selected. In this paper the results related to seed yield and some other characters connected with it are presented.