I observed a pair of Olive-backed Euphonias (Euphonia gouldi) in the Caribbean lowlands of Costa Rica singing a prolonged antiphonal duet on 28 February 2006. The male and female were perched near one another within a larger conspecific flock, and exchanged closely-coordinated notes in phrases lasting ∼5 sec. This appears to be the first report of duetting behavior for this species or for any member of family Fringillidae.