Juvenile green turtles occupy home ranges on shallow reefs that parallel the southeastern coast of Florida. When disturbed, the turtles often flee eastward toward deeper water. We captured turtles at night and recorded their orientation during the next 2 days while they swam tethered inside a large circular outdoor pool and were exposed to the sun. The turtles from 2 reef sites were significantly oriented eastward. After 7–10 days of exposure to a laboratory photocycle advanced by 7 hours, turtles tested in the same pool were significantly oriented westward, the predicted direction if they used the sun for orientation. Westward orientation was unaffected by placing either disc magnets or brass discs of identical mass above the turtles' heads. These results are consistent with the hypothesis that orientation under clear skies is preferentially based upon solar cues. However, control turtles exposed for 7–10 days to a photocycle advanced by 1 hour were also expected to swim generally eastward, but, for unknown reasons, failed to show significant orientation. Our results therefore support the hypothesis that the sun is used as a compass, but additional experiments will be required to provide definitive evidence.

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