Behavioral observations on a large group of free-ranging rhesus monkeys (Macaca mulatta) were carried out on Cayo Santiago from January I968 through December I969 and from October through December I970. Records of mating activity (single mounts, mount series, copulations) indicated that genealogical mating activity on Cayo Santiago had the following characteristics : (I) 5.4% of all recorded mount series and copulations involved mother-son pairs; (2) mating was observed in 3I% of 26 individual pairs of mothers and sons; (3) mating was seen in I2% of 42 individual pairs of brothers and sisters; (4) of I0 instances of mother-son mating, seven involved pairs in which the mother was dominant to the son; (5) mating in mother-son and brother-sister pairs occurred in high and low ranking genealogies within the group; (6) with two exceptions, all three to five-year old males who did engage in sexual activity with their mothers did not mate with non-related females during that breeding season; and (7) with one exception, all instances of mother-son and brother-sister mating involved males from three to five years of age. These findings are in sharp contrast to observations made on a smaller group on Cayo Santiago. Possible causes for this difference between social groups of different size are discussed.