1. The behaviour of flatfish, haddock and sandeels near Danish seine nets was recorded by divers using cameras and tape-recorders. 2. Flatfish swam at right angles to moving seine net ropes and collected in front of the net groundline by a zigzag process. 3. The density of flatfish in the net mouth rose sharply when the net was about half closed, and then declined more gradually. 4. The duration of swimming of flatfish was not directly related to the speed of movement of the gear. 5. Haddock were herded by the ropes and appeared near the net much later than flatfish. When in the net mouth they showed an optomotor reaction to the net wings. 6. The duration of swimming of haddock in front of the net was not directly related to its speed. 7. Sandeels avoided the headline of the net but swam through the netting. Near the sea-bed they behaved like haddock.