1. The swimming speeds attained by hatchery-reared plaice larvae have been determined by measurements made (a) in still water, and (b) against a current flowing through a long glass tube.
2. In still water contact stimuli were used to initiate swimming. No optomotor response was found, and it was not possible to use the ‘fish-wheel’ of Bainbridge (1958).
3. Larvae in the experimental tube stemmed the current for brief periods only, their position then being near the periphery of the upper quadrant. The speeds were found to show a linear relationship to larval length multiplied by the height of the post-anal musculature. Observations in still water show that these values correspond to those of steady swimming: they are not maximum speeds.
4. Speeds ranged from about 1.5 cm./sec. when newly hatched to 9 cm./sec. in late larvae. Darting velocities of up to 15 cm./sec. were recorded with half-grown (stage III) larvae. In such larvae cruising speeds were about three fish-lengths (3L) per second, and the maxima about 10L/sec.
5. The design of gear for collecting fish larvae at sea is discussed. Only a high-speed tow-net, towed at about ten times the velocity attainable by the largest larvae and giving almost no warning of its approach, can be considered to collect a quantitatively balanced sample.