1. An apparatus is described to study the response of fish larvae to water currents.
2. Close to the wall of the water tunnel all pelagic larval stages of the plaice showed a typical rheotropic response.
3. Vector analyses of orientation showed that the response occurred only within 3 cm. from the wall and was absent in midwater.
4. Although the ability to swim against the current increased with age, significant upstream movement only appeared in stage III larvae. Laterally flattened stage IV larvae were less able to swim upstream, but on touching the bottom lay on their sides and showed the behaviour of fully metamorphosed stage V larvae and older fish.
5. In dim light (> 2·5 m. candles) orientation to the current at the side of the tank also disappeared, but observations with an infra-red viewer and source showed that larvae which touched the wall in the dark could still orientate.
6. It was concluded that the mechanism of the rheotropic response was primarily visual, although it could also be produced by tactile clues. There was no evidence that larvae could respond to the velocity gradients at the tank wall.