1. A flume is described for producing currents of sea water for behaviour studies of marine fish. Its cross-section is 30 cm. wide x 32.5 cm. deep and the observation length is 550 cm.
2. Velocity and depth of water can be finely controlled and the maximum velocity in the level position is 69 cm./sec. with a depth of 10.8 cm. Maximum depth is 29 cm.
3. By tilting the flume the maximum velocity can be increased to 132.5 cm./sec. at a depth of 5.5 cm. with a maximum gradient of 1 in 40.
4. An instrument for measuring current velocity by an optical technique is also described. Its range is 0.8-240 cm./sec. with an accuracy of ± 1% over the range 0.8-8 cm./sec. in its lowest gear and ± 0.5% from 2 to 240 cm./sec. in the higher gears.