Ultraviolet radiation, UV-B (280-320 nm), has harmful effects on marine phytoplankton. The presentexperiments were done to determine the effects of UV-B radiation on motility and growth of four marine motile dinoflagellates, Gyrodinium aureolum, Prorocentrum minimum, Heterocapsa triquetra and Scrippsiella trochoidea. The specific growth rate of the dinoflagellates showed an inhibition after 1 hour UV-B (1.5 kj m(su-2}day-1) daily during 1 week. The motility of H. triquetra showed a pronounced inhibition after 12 kJ m-2day-1, the motility of S. trochoidea did not show the same sensitivity to UV-B radiation. The photosynthetic inhibitor DCMU (10-5M) inhibited the motility of S. trochoidea but not the motility of H. triquetra. Action spectra (UV-B, 280-320 nm) for inhibition of motility of G. aureolum showed the maximum sensitivity at 280 and 290 nm.