Our aim was to describe changes in ambient water temperature during the course of migration by northern elephant seals (Mirounga angustirostris) and to examine evidence for the seal using abrupt temperature gradients for locating prey. During migration in the post breeding season, the diving patterns of 10 adult females and 7 breeding-age males from Año Nuevo, California, were recorded with time--depth recorders in 1989--1991. Recorded sea surface temperatures declined from 11--13 °C to a low of 3--9 °C as the seals moved north and increased as they returned. Depth of diving was not closely linked to sharp thermal gradients. A thermocline was evident only at the beginning and end of the migration in less than 100 m of water, where less than 2% of diving takes place. There were sex differences in the temperature range at the depths where 75% of diving and foraging occurred, owing in part to habitat separation. The temperatures were lower and the range narrower for females (4.2--5.2 °C at 388--622 m) than for males (5.3--6.0 °C at 179--439 m). We conclude that the northern elephant seal habitat does not provide abrupt changes in temperature that might serve as important cues for locating prey.