1. Differential male reproductive success is commonplace in mammals and frequently attributed to variation in morphological traits that provide individuals with a competitive advantage in female defence mating systems. Other mammalian mating systems, however, have received comparatively little attention and correlates of male reproductive success in them are less well understood. 2. We studied a free-ranging population of North American red squirrels (Tamiasciurus hudsonicus Erxleben) exhibiting year-round individual territoriality. Males must temporarily vacate their territories to locate spatially dispersed receptive females, thereby setting the stage for a scramble competition mating system. 3. We predicted that both male annual mating success (measured as the number of females copulated with) and annual reproductive success (measured as the number of offspring sired) would be positively correlated with both search ability (measured as the number of oestrous females located over the mating season) and effort (measured as mating season home range size), generating directional sexual selection on these two metrics. 4. Mating season home ranges of males showed, on average, an almost 10-fold increase relative to those measured during the nonmating season, while those of females showed a more moderate twofold increase and both annual mating and reproductive success of males was positively correlated with search ability and search effort. 5. The spatial dispersion of females, resulting from the strict territorial social structure of red squirrels, gave rise to a predicted scramble competition mating system. Furthermore, the strength of sexual selection on behavioural traits in this mating system equalled previous estimates for morphological traits in female defence mating systems.