Black and brown individuals of a series of litters of house mice, belonging to 3 genetically distinct stocks, were compared between the ages of 30 and 180 days. Brown [male][male] averaged 3.9% heavier than black [male][male], and brown [female][female]averaged 3.1% heavier than black [female][female] as adults. The growth of brown mice was more rapid than that of black ones at all of the ages when measured. The difference is considered to be due to the physiological effects of the brown gene upon the growth of the mouse.