I observed the incubation and nestling periods of five nests of the Violet Sabrewing (Campylopterus hemileucurus) in Costa Rica. Each nest was built above running water. Mean incubation period was 20 d and only the female incubated. Mean nestling period lasted 23 d. Nestlings reached and surpassed adult mass before fledging (132% of adult size in males and 130% in females), although wing, tail, and culmen length did not reach adult size until after fledging. Because the Violet Sabrewing is highly sexually dimorphic in size and plumage color, I compared the postnatal development of five females and two males. Males were heavier than females and grew more slowly, consistent with generally slower growth rates in larger birds. The T_(10–90) period was 15.3 d for the fastest growing male and 13.8 d for the fastest growing female.