Knowledge of the determinants of reproductive success is essential for understanding the adaptive significance of particular traits. This study examined whether particular behavioural, morphological, physiological or genetic traits were correlated with male dominance and reproductive success using three semi-free ranging captive populations (n = 98) of the eastern grey kangaroo (Macropus giganteus). Morphological traits measured included body weight, head, forearm, tail, pes and leg length, forearm and bicep circumference, and testis size. Blood samples were collected to determine serum testosterone concentrations. All individuals were typed for ten microsatellite loci and paternity determined for each pouch young. To determine the influence of relatedness and genetic diversity on male reproductive success, internal relatedness, standardised heterozygosity and mean d2 were calculated. Dominant males sired a significantly higher proportion of more ooffspring than smaller, lower ranked males, and had higher testosterone concentrations. Males that sired offspring were significantly heavier and had larger body size. Sires were significantly more heterozygous and genetically dissimilar to breeding females, than non-sires. Despite the wealth of knowledge on the social organisation of kangaroos, this is the first study to assign examine parentage and male reproductive success using molecular evidence, as well as its influence on their mating system.