The relationship between sperm competition and behavioural forms of male competition was investigated by comparing paternity of litters when female meadow voles, Microtus pennsylvanicus, were exposed to two males sequentially (sperm competition only) or simultaneously (potential interaction between sperm competition and mate guarding). Voles were descended from two populations: one with naturally occurring blond voles (homozygous recessive) and the other with voles homozygous for the dominant dark coat. In sequential tests, blond females received one ejaculation each from one blond and one dark male (order of males varied among females). There were no mating-order effects, but dark males sired more pups. Because this differential fertilization may have masked mating-order effects, we repeated the experiment with voles homozygous for different transferrin genetic variants. Again, there were no significant mating-order effects. When females were exposed to both males simultaneously, the aggressive blond males sired more pups because they mated more and effectively guarded females, thus preventing devaluation of their sperm by other matings. The more fertile dark males only sired pups if they mated first, before the blond males encountered the pair (a potential problem in field data on sperm precedence if more fertile individuals consistently mate either first or last). Mate guarding should reduce sperm competition from other males in high-density populations. In contrast, finding and fertilizing females may be more important in low-density populations, conclusions consistent with these population densities for several years prior to capture.