The influence of larva1 behaviour on the ecology of the athecate hydroid Clava multicornis was examined. Planulae were positively phototactic, positively schizotactic and settled away from other conspecifics. Planulae were indifferent to the roughness, relief and inclination of the substrate. Currents ranging from 2 mm/s to 160 mm/s did not detach them from alga1 fronds. Larva1 attachment and metamorphosis occur at low tide and are induced by the surface films covering the brown algae Ascophyllum nodosum and Fucus vesiculosus. The behaviour of the planula larvae determines the spatia1 distribution of the hydrozoan colonies in the White Sea intertidal zone.