Normal swimming behaviour of Lytechinus pictus larvae and the effects of selected drugs are described, based on direct observation and videotapes of free-swimming and tethered larvae. The principal effector response is a coordinated ciliary reversal that enables larvae to back away from obstacles and avoid entanglement. The effect is best seen in the epaulettes, whose large size makes the pluteus an especially favourable subject for behavioural observation and tests. Reversals and sustained arrests can be induced by various cholinergic agonists, notably nicotine, which is active to concentrations of 0.2 <latex>$\mu $</latex>M. Dopamine and adrenaline cause reversals and arrests as well, but they act initially on the response as a whole, increasing its frequency, rather than directly on ciliary beat. The data suggest a two-step control sequence, with an initial catecholamine-dependent step that triggers a cholinergic effector response.