Females are larger than males in more species of mammals than is generally supposed. A provisional list of the mammalian cases is provided. The phenomenon is not correlated with an unusually large degree of male parental investment, polyandry, greater aggressiveness in females than in males, greater development of weapons in females, female dominance, or matriarchy. The phenomenon may have evolved in a variety of ways, but it is rarely, if ever, the result of sexual selection acting upon the female sex. The most common selective pressures favoring large size in female mammals are probably those associated with the fact that a big mother is often a better mother and those resulting from more intense competintion among females for some resource than among males. It appears that, in general, more than one such pressure must affect the females of a species, and that their combined effects must not be countered by even stronger selective pressures favoring large size in males, before the result is that of larger size in the female sex. Sexual selection may often be operating upon the male sux in mammals even when it is smaller. Present knowledge about the species of mammals in which females are lager than males is quite rudimentary. Much more information is needed before we will be able to speak of the selective pressures accounting for the phenomenon with any reasomable degree of certainty. Perhaps the most fruitful approach would be a series of field studies of groups of related species in which females are larger in some species and males are larger in others.