Yellow-lipped sea kraits (Laticauda colubrina) are large (to 1.5 m, 2 kg) amphibious sea-snakes that forage for eels in tropical oceans but return to land to digest their prey, slough their skins, mate, and lay eggs. During three-month field seasons in two successive years, we quantified various aspects of the behaviur of sea kraits on a small island off the coast of Viti Levu, Fiji. Radiotransmitters were surgically implanted in 16 snakes, and regular surveys were conducted to quantify the times and places of various activities by nontelemetered snakes. The radio-tracked snakes spent equal amounts of time on land versus in the ocean, moving between these two habitats about once every 10 days. Their mean duration of time on land fits well with the time required for sloughing and digestion, as measured in outdoor enclosures. These snakes maintained relatively high and constant body temperatures both while on land and in the water; the only overt thermoregulation involved shade-seeking. Different age and sex groups were active in different places and at different times of day. For example, juvenile sea kraits rarely ventured far from water, whereas adults often moved well inland. The snakes moved about at night, engaged in courtship during the morning and were inactive during the afternoon. Thus, snakes were frequently found in courting groups during the morning, but most were solitary at night. Movements between land and sea generally occurred at night on gently sloping areas; movements of adult male snakes also were affected by tidal conditions and by the presence of females. Male sea kraits move about more frequently and actively on land than do females, in keeping with sex differences in locomotor performance.