We documented the juvenal plumage of the Green-breasted Mountain-gem (Lampornis sybillae) during mist-netting operations in the cloud forest at La Tigra National Park, Honduras from February to April 2006. A recently-fledged juvenile of this species was caught on 17 March and, contrary to previous suggestions, we found the throat and breast were mottled green. Ninety-eight immature mountain-gems intermediate between this juvenal plumage and that of adults were also caught during our study. Both males and females of the Green-breasted Mountain-gem apparently begin replacing juvenal throat feathers soon after fledging and prior to molting flight feathers. A high capture rate of young hummingbirds at the end of the dry season, including recent fledglings and individuals showing only traces of juvenal plumage, suggests a protracted breeding season that we estimate to last at least from November through March. We also caught adults in a variety of stages of flight-feather molt, perhaps part of a transition from breeding; molting in our population is estimated to span at least an 8-month period.