In reptiles, phenotypic measures such as body size usually predict a male’s success in territorial interactions. Recent evidence from fish, birds, and mammals has shown that genetic heterozygosity also has a strong influence on competitive ability and territory quality. Here, we provide a comprehensive assessment of the social structure and factors affecting male territory quality and aggressive behavior in a dense population of Tuatara, a long-lived reptile that maintains long-term territories, on Stephens Island, New Zealand. The only significant predictor of female access and competitive ability was male body size, and there was no relationship between male body size or condition and individual genetic heterozygosity. Body size, body condition, and heterozygosity did not predict territory size. Also, heterozygosity, body condition, and territory size had no relationship with the number of females to which a male had access. Large males were more effective at (1) monopolizing areas where females were most dense and (2) guarding females by consistently winning aggressive encounters with other males. Our finding of no relationship between territoriality and heterozygosity probably reflects the genetic background of this large, outbred population or that behavioral attributes or neutral heterozygosity are not appropriate individual fitness correlates for these long-lived reptiles.