A group of 1,052 C57BL/6J mice (296 males and 756 females) was kept under well-defined, clean laboratory conditions from the age of 6 weeks until natural death. The survival curves of males and females (computer-produced 3, 4, and 5 parameter curves, Gompertz curve histogram) were established and shown to follow a logistic function. The average life-span amounted to 878 plus or minus 10 days for males and 794 plus or minus 6 days for females. These values distinctly exceed comparable values given in the literature. They are attributed to favorable conditions of animal care and to supposed alterations in genetic background. A genetic drift in sex-dependent mean survival time occurred in the genetically unstable C57BL/6J strain between 1966 and 1970. Before this drift, the males died sooner; after it, they lived longer.