1. Observations were made on the reactions of plaice (2-28 cm. length) to water currents in a small flume (30x325x550 cm.), and a behaviour pattern is described.
2. At a threshold velocity fish turned to face upstream, usually showing burying movements or a clamped-down posture. With increasing current velocity they remained on the bottom, showing a posterior fin-beating response. Fish displaced by the current reacted firstly by moving upstream on the bottom and re-settling and at higher velocities by bursts of swimming in midwater. A velocity was finally reached at which each fish was displaced downstream over the weir.
3. On a sandy bottom small buried plaice did not usually react to the current until the covering layer of sand was eroded away.
4. Observations with an infra-red viewer showed that although the typical optomotor response shown in midwater disappeared in the dark, behaviour on the bottom remained the same as in the light.
5. The lowest velocity at which fish turned to face the current was 0.8 cm./sec. and most fish orientated at a velocity below 8 cm./sec.; there was no increase in threshold velocity with length. There was great variation in the velocity at which individual fish were finally displaced, but there was a general increase with length from 50 to 70 cm./sec.
6. A displacement velocity was found for anaesthetized fish (24-30 cm. length) heading upstream, which was significantly higher than for those heading downstream.
7. Photographs of the flow patterns over live fish on the bottom showed that they were most effectively streamlined when heading directly upstream. Injection of dye showed that water penetrating beneath the fish was ejected by the posterior fin-beating response, and that the operculum of the ocular surface only was used to exhale water from the gills.
8. Experiments with water jets demonstrated three areas whose stimulation elicited a behavioural response shown in the current. These were the leading edges of the body on either side of the head, including the right nasal papilla, and the tail.
9. The results are discussed in relation to the ecology of the plaice and the contranatant theory of fish migrations. It is concluded that the observed behaviour is hydrodynamically advantageous in maintaining station on the sea-bed.