HATCHLING sea turtles usually emerge from their nests at night. The hatchlings are buried in the sand and so they cannot use light as a cue. Hendrickson1 has suggested that during the day heat inhibits activity and keeps the turtles below the surface until the cool of night. Previous work has supported this idea in a general way: green sea turtles, Chelonia mydas, tested 2–3 days after emergence, gave sharp decreases in the type of photic activity involved in the journey from the nest to the sea as the ambient temperature was increased over the 21–26° C range2. But to show that in nature temperatures actually limit the emergence of hatchlings to night-time it is necessary to relate the level above which activity declines to the temperatures prevailing by day on the nesting beaches. Furthermore, in order to check that activity tests made in controlled experimental conditions reflect activity in the natural situation, information on the temperatures of nests from which turtles are in the process of emerging is desirable. Accordingly, three new sets of measurements were made.