Seven years' detailed behavioural observations on a small population of adders in southern Sweden provided data for a quantitative analysis of the behavioural determinants of male mating success. Male adders are subject to strong intra-sexual selection. The number of matings obtained by a male adder was enhanced by higher male mobility (distance travelled during the breeding season), mate-finding ability (females located per metre travelled), and ability to defeat rival males in combat bouts (proportions of bouts won). In combination, variation among males in these three abilities accounted for more than half of the variation documented in male mating success. Male success rates in combat bouts were strongly dependent on body size (and hence, age), and reproductive tactics shifted concomitantly. Small males fought other males (especially, large males) only rarely, and achieved the majority of their matings in the absence of other males. Larger males fought other males more frequently, and displayed mate-guarding behaviour on some occasions. Although mating success of male adders was strongly enhanced by larger body size, age per se appeared to exert no independent influence on mating success. Overall, the mating system of adders impose strong sexual selection on a male's ability to locate females and to conquer rival males in battle.