Sixteen silver eels (Anguilla anguilla L.) were tagged with pressure-sensing ultrasonic transmitters and tracked for 1 to 152 hours, 12 in the western Mediterranean and 4, treated with pituitary extracts, in the Sargasso Sea. When released on the shelf of the Mediterranean near Gibraltar the eels preferred a course which led them continuously to deeper water. Four specimens released on the Morrocan shelf, reached depths of 350 m, continued to swim north or changed their earlier easterly course towards the north. Three eels in the Sargasso Sea showed progress in a southwesterly direction. No influence of tidal stream on the swimming course and speed is obvious except for one eel, released in the Straits of Gibraltar. In the Mediterranean, the eels exhibited conspicuous daily vertical migration downward during dusk and upward during dawn with a mean swimming depth of 196 m during darkness, 344 m during daylight. Depth preference during night probably varied with the phase of the moon. Maximum swimming depth near Gibraltar as well as in the Sargasso Sea was nearly 700 m. The eels in the Mediterranean swam at a mean speed of 0.3 m.s−1 and were slower than most eels tracked in northern waters. Differences in the speed and in the directed swimming in the course of the lunar or the solar day are visible.