Behavioural ecologists have long been interested in mating systems and variance of reproductive success. Highly variable molecular markers now enable researchers to reassess mating systems from the genetic point of view. We used 10 microsatellite loci to detail the mating pattern and male reproductive success in a natural population of the common vole Microtus arvalis, one of the most numerous species in Europe. By genotyping 32 females and their offspring, we found evidence for multiple paternity in 50% of litters sired by two or three males. This result was confirmed by paternity analysis of candidate fathers caught in the population; it also showed that both males and females mated with several unrelated partners. Comparisons of two sires in a given multiple-sire litter showed their relatedness to be low. The common vole population was characterized by a relatively high standardized variance of male reproductive success, indicating that males competed for mating. While one of the males could sire up to 83% of offspring in a multiple-sire litter, mating with an already mated female gave lower reproductive success than mating with one female exclusively. Our results suggest that the occurrence of multiple paternity in the common vole population can be explained by the inability of males to monopolize and mate with all females of a colony, and also by their tendency to increase their reproductive success by getting access to already mated females.