Molecular analyses have confirmed that the majority of socially monogamous birds follow a genetically promiscuous reproductive strategy. Most such studies, however, have addressed species of the North Temperate Zone; tropical species are grossly underrepresented, although most of Earth's biodiversity is concentrated in the tropics. In this study, we describe the genetic mating strategy of the Rufous-and-white Wren (Thryothorus rufalbus), a socially monogamous duetting neotropical songbird. Over 8 years of studying a marked population in Costa Rica, we recorded genetic data from 51 broods. Microsatellite analysis of four variable loci revealed that the Rufous-and-white Wren follows a mixed reproductive strategy, where extra-pair young are found in a small minority of broods. Three of 158 nestlings (2%) were the result of extra-pair fertilizations; three of the 51 broods (6%) contained a nestling with alleles mismatched to its social father. We assigned paternity of two of the extra-pair nestlings to males in territories adjacent to the cuckolded males. During behavioral observations, we observed within-pair copulations rarely and extra-pair copulations never. The rate of extra-pair paternity we report here is lower than the average reported for socially monogamous birds in the North Temperate Zone but comparable to rates in three closely related tropical wrens. Rufous-and-white Wrens are renowned for their coordinated vocal duets. We compared rates of extra-pair paternity among socially monogamous tropical birds, contrasting four duetting species against twelve nonduetting species and found lower levels of extra-pair paternity in the former group, suggesting that duetting and low levels of extra-pair paternity are related.