We observed a case of infant adoption in an unprovisioned group of wild black-fronted titi monkeys (Callicebus nigrifrons). During our long-term field study we noticed that an infant had moved from one of our study groups ("Desbotado") to another ("Rio"). Observations of the adoptive group confirm that it was being cared for by the adult male, and initially the group's adult female was nursing the infant alongside her biological infant. Interestingly, the native and adoptive groups have frequent inter-group interactions, but at no point have we observed the native group trying to retrieve its infant. As of April 2007 the infant has been living in its adoptive group for 19 months. These data document the first case of adoption in this genus; they suggest that infant recognition is poorly developed in this species and that under certain circumstances wild groups of C. nigrifrons can successfully rear twins. In our study population reproductive females give birth to one infant every year; the only case when this has not happened is with the group that adopted the infant, suggesting that adoption may generate a reproductive cost.