From Long Island, N.Y., to Rio de Janeiro, the Ghost Crab, Ocypode quadrata, is found on sandy beaches. Behavior of specimens from Townsend''s Inlet, N.J., was studied by day and night through the summer and autumn, and specimens were observed for extended periods in captivity. Adults make burrows in the dry beach beyond storm-wave lines and in the dunes, emerging principally at night when they obtain food from the beach drift and wet their modified gill chambers. Females well loaded with eggs were observed in late July. At this time and until Oct., a great variation in size was noted among specimens on the beach, probably indicating that the aquatic megalops stage transforms to the terrestrial crab throughout the summer. In Oct., burrows were higher on the beach and much longer, but specimens were active during a warm spell; apparently the species hibernates in these burrows (which do not reach the water or even wet sand). Heat tolerance is high, but in July exposure for 2 hrs. to 12[degree]C was fatal. Methods of combat and concealment were studied, and differences noted in behavior of the sexes. Males resist being driven into the water, but [female][female] readily wade out beyond reach. While completely submerged in quiet clear water, gravid [female] force a current through their eggs by opening the abdominal fold slightly and quickly rotating the body on the bases of the ambulatory legs until the dorsal surface of the carapace is downward. A similar attitude is struck by both sexes in attempting to seize in the chelipeds an enemy which is above them. Normal running on the beach involves all 8 ambulatory legs, but rapid running to escape capture involves only 6 legs, the hind pair being held clear of the ground; additional maneuverability seems to be gained in this way. Captive specimens accepted cold sea water from a pipette, as well as animal matter (dead or alive) including dried insects, but no Ghost Crab would accept fireflies.