Reindeer (Rangifer tarandus) is considered one of the most polygynous ungulates, so high variance in male mating success is expected. We assessed the variance in male mating success in a wild reindeer population with natural adult sex ratio in order to test this assumption. We obtained samples from 321 calves from the same year class and from 206 adult males. These samples represented an estimated 84% of the potential sires in the population. By analyses of 23 microsatellite loci and use of a likelihood ratio approach, we assigned 49% and 90% of the calves a sire at 95% and 80% confidence interval, respectively. Approximately 50% of the assigned calves had 1 or no paternal half-sibs, and approximately two thirds of the males contributed genetically to the cohort. At 80% confidence interval, 2.9% of the males sired >7 offspring, and 20 calves were assigned to the most successful male. Mean and variance in the number of calves assigned by different male age classes increased with age, although several of the yearling males were more successful than some of the adults. Our findings did not support the traditional assumption that most reindeer calves were fathered by a small group of highly successful males, suggesting that alternative mating strategies may play a more prominent role in reindeer than previously expected. Within a sustainable management concept, our findings demonstrate the importance of maintaining a reasonable proportion of young and adult males in the population.