Mice from the inbred albino strain, ST/bJ, were crossed to mice from the inbred black-agouti strain, CBA/J. An F-1 and F-2 were obtained. From the F-2, 2 lines were established: one homozygous for albinism and the other homozygous for pigmentation. Twenty male mice from the derived albino line were randomly paired with 20 males of the derived pigmented line. Each pair was then placed in a cage. Into 10 of these pens, a derived albino female was placed, while into the other 10 cages an inbred albino, ST/bJ, female was placed. Ten litters were obtained from each pen giving a total of 200 litters. All males were proven fertile both at the beginning and at the end of the experiment. In the pens where the derived albino females were used, the derived albino males fathered 54% of a total of 653 mice born. In the cages where inbred albino, ST/bJ, females were used, the derived albino males sired 63% of a total of 586 mice born. These differences in reproductive success were due to the differences in composition of litters fathered by both competing males (mixed litters). The results are discussed with relation to the possible roles of the gene for albinism, and selective fertilization in sexual selection.