I examined the ecological characteristics of dipteran larvae infesting Leatherback seaturtle nests on Gandoca Beach (9°35'N, 82°34'W), Costa Rica. Over 74% of nests were infested by fly larvae. Flies acted as nest scavengers, but 44 live hatchlings were attacked indicating that flies also act as opportunistic predators. Several dipteran species were recorded, with the sarcophagid ' Eumacronychia sternalis' Allen being dominant. Infestation by ' E. sternalis' happened shortly after hatchling emergence, suggesting that flies are attracted to emanating odours of decomposing nest material brought to the surface by emerging hatchlings. Sampling year, bacteria-fungus invasion, and the interaction between nest depth and the number of dead hatchlings best predicted the incidence of larvae within nests. Baskets covering clutches translocated to egg hatcheries were effective in reducing infestation. My results suggest that flies do not seriously threaten Leatherback populations on Gandoca Beach, but that flies occasionally kill moribund hatchlings.