The effects of solar ultraviolet radiation (UV) on carbon uptake, oxygen evolution and motility of marine phytoplankton were investigated in coastal waters at Kristineberg Marine Research Station on the west coast of Sweden (58° 30′N, 11° 30′E). The mean irradiances at noon above the water surface during the investigation period were: photosynthetic active radiation (PAR, 400–700 nm) 1670 μmol m−2 s−1; ultraviolet-A radiation (UV-A, 320–400 nm) 35.9 W m−2 and ultraviolet-B radiation (UV-B, 280–320 nm) 1.7 W m−2. UV-B radiation was much more attenuated with depth in the water column than were PAR and UV-A radiation. UV-B radiation could not be detected at depths greater than 100–150 cm. Inhibition of carbon uptake by UV-A and UV-B in natural phytoplankton populations was greatest at 50 cm depth and the effects of UV-B were greater than those of UV-A. At depths greater than 50 cm there was almost no effect of ultraviolet radiation on carbon uptake. PAR, UV-A and UV-B decreased oxygen evolution by the dinoflagellate Prorocentrum minimum. Inhibition of oxygen evolution was greater after 4 h than 2 h but it was not possible to distinguish the negative effects of the different light regimes. The motility of P. minimum was not affected by PAR, UV-A and UV-B. The importance of exposure of phytoplankton to different light regimes before being exposed to natural solar radiation is discussed.