Summary. A low fertility of DDK females mated to males from other inbred strains of mice, revealed in preliminary inter-strain crosses of several strains, was confirmed in a detailed study of strains DDK, KK and NC, the nine possible matings being arranged. The mean litter sizes of four types of crosses, DDK females × KK males, DDK females × NC males, KK females × DDK males, and NC females × DDK males were 1·1, 2·6, 7·2 and 7·7 respectively. The mean litter sizes of the three inbred strains, DDK, KK and NC, were 5·8, 5·6 and 7·0 respectively. The lowered fertility of DDK females mated to KK and NC males was accounted for by embryonic losses which occur about 3 to 4 days after copulation. The embryos die just before or in the process of implantation, i.e. before or during the process of blastocyst formation. Probable causes of the embryonic losses in DDK females mated to KK and NC males are discussed.