ABSTRACT:  In laboratory experiments, we determined pivotal temperature (29.6°C), i.e. the constant temperature yielding 50% of each sex, and pivotal incubation duration (57.3 d) for hawksbill turtles Eretmochelys imbricata (Agassiz, 1857) nesting on Mona Island, Puerto Rico. Based on the laboratory curves for sex ratio, predictions were made about sex ratio in field conditions. Temperature of natural nests provided a better prediction of the actual independently verified sex ratios than incubation duration. Predictions made from temperature were not highly sensitive to the exact specification of the thermosensitive period within incubation. Recommendations are made for estimating sex ratios of hatchlings on beaches for which there is currently no information. The pivotal temperatures for Mona Island hawksbill turtles were similar to those obtained previously for hawksbills nesting in Antigua and in Brazil, suggesting some conservatism in this characteristic. If pivotal temperatures are similar for different areas, then sex ratios of turtles hatching on thermally different beaches are likely to vary considerably. Implications for the impacts of global warming on sea turtles are also discussed.