Breeding, male North American sunfish (Centrarchidae), are often brightly coloured and promiscuous. However, the largemouth bass (Micropterus salmoides) is sexually monomorphic in appearance and socially monogamous. Unlike some other nest–tending centrarchids in the genus Lepomis, largemouth bass have also been reported to provide biparental care to eggs and fry. Here we use microsatellite markers in order to test whether social monogamy predicts genetic monogamy in the largemouth bass. Offspring were collected from 26 nests each usually guarded by a pair of adults, many of which were also captured. Twenty–three of these progeny cohorts (88%) proved to be composed almost exclusively of full–sibs and were thus the product of monogamous matings. Cuckoldry by males was rare. The genetic data also revealed that some nests contain juveniles that were not the progeny of the guardian female, a finding that can be thought of as low–level ‘female cuckoldry’. Overall, however, the data provide what may be the first genetic documentation of near–monogamy and biparental care in a vertebrate with external fertilization.