The effects of social dominance on male mating behavior and paternity in a troop of rhesus monkeys (macaca mulatta) were examined. A wild-caught troop of monkeys, captured in India in 1972, has been monitored in captivity for eight years. Some animals have been culled from this troop, but no new animals have been added except through births. The social structure of the troop has remained fairly stable over the eight years, although the number of adult males at any given time has ranged from four to ten. We blood typed 77 offspring born between 1973 and 1980 that survived to two months of age and determined paternity for 48 of them. During the eight-year period, the dominant male sired only 13% to 32% of the offspring, even though he participated in 67% of the observed copulations. In contrast, the second ranking male sired 30% to 48% of the offspring, but participated in only 14% of the observed matings. The frequency and duration of copulation in any one year appeared to reflect a male's rank in the dominance hierarchy. However, in all but one year of our study, the largest number of offspring were sired, not by the dominant male, but by young males who were second or third in rank. Nevertheless, the dominant male may have a selective advantage because he sires a large number of offspring early in life, and continues to produce offspring over many years. This study demonstrates that observed copulations are imprecise indicators of paternity, and that paternity in any one breeding season is a poor indicator of the genetic structure of a population.