We studied genetic similarity between adults and nestlings in putative social families (i.e., 2 adults and a chick) of a seabird that provides obligate biparental care, the Australasian Gannet (Morus serrator), in New Zealand. We detected DNA fingerprint mismatches in 12% of 26 nests between the chick and 1 of the 2 attending adults sampled. No parent–offspring genetic mismatch was detected in nests with 4-week-old or younger and sedentary nestlings, whereas adult–nestling mismatches were detected only in nests with 5-week or older and more mobile young sampled. We conclude that the genetic mating system of this sulid species is predominantly monogamous.