Previous studies on parental and spacing behavior of Microtus pennsylvanicus suggest a promiscuous mating system, but attempts to find multiple paternity in single litters have been unsuccessful. In this paper we present evidence of multiple paternity in single litters conceived in the wild early in the breeding season. The proportion of litters sired by multiple males was estimated, by a conservative method, to be 33.1%.We argue that the presence of promiscuity, rather than polygyny, in M. pennsylvanicus is the result of two factors. First, overwintered breeding males are similar in age and size, resulting in small variation in competitive ability among males. This reduced variation in competitive ability reduces the possibility that some males defend several females and others defend none. Second, the habitat structure of the meadow vole makes it difficult for a male to detect other males nearby, and this reduces the possibility that one male excludes others from mating when a female comes into estrus.