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Wild house mice, Mus musculus L., were trapped on a farm, and bred in the laboratory at two environmental temperatures, 21°C and −3°C. The reproductive performance of 12 pairs of the second generation reared in the laboratory was recorded to the age of two years; ten pairs of the first generation were transferred to −3°C at mating, and were similarly observed for two years. The wild mice at 21°C resembled inbred laboratory mice in litter size and the weight of young at three weeks, but produced many more litters: nestling mortality was about 16%. The fertility of the wild mice was, however, inferior to that of some outbred laboratory mice. At −3°C the number of litters per pair was about half that at 21°C, and nestling mortality was over 50 % losses were mainly of whole litters. Litters at birth tended to be larger at −3°C; the young aged three weeks were heavier in the cold environment. At birth, fifth litters were largest at 21°C, and fourth litters at −3°C. Weights of individual young aged three weeks tended to rise with parity in both temperatures. Some mice, in both temperatures, continued to produce litters for the full two years.