Studies of deer mouse populations in nature have recorded significantly more males than females (.005 > P). Since most studies have utilized trapping techniques, greater wandering tendencies of males and consequently greater trap exposure have been suggested to explain the excess males recorded. Tabulation of the sex ratios at birth of more than 500 litters from a laboratory colony of Peromyscus maniculatus bairdii indicate significantly more males than females produced (.025 > P). Excess production of males has also been recorded for freely growing laboratory populations of the same species. Peromyscus therefore, appears to be an additional genus exhibiting aberrant sex ratios.