In addition to the normal dark brown old-field mice, a pale brown phenotype occurs on the Savannah River Plant in South Carolina. A linear model can be used to describe the reflectance spectrum of the dorsal pelage of the two color variants. The reflectance spectrum of pale brown adults had a higher intercept and slope than the darker adults. The paler mice occurred in high frequency only on the lighter, Lakeland Sand soils. Pleiotrophic effects of the autosomal pale brown gene were noted in reproductive performance in the laboratory, trappable life span, and number of captures per unit time, but not in the growth rate of the two variants. Under field conditions, presence or absence of supplementary food also modifies the expression of the phenotype. The complex interaction of genetic and environmental factors and inadequate knowledge about the environment of the two phenotypes makes their distribution difficult to interpret. It is apparent, however, that the simple hypothesis of predators selecting mice that contrast with their background is not sufficient to explain all of the data.