Several unicellular algae were exposed to artificial UV-B (280-320 nm) radiation after adaptation to high (43 W m-2) and low (19 W m-2) visible light. UV-B radiation had different effects on rates of photosynthesis. motility and absorption spectra for these species. Photosynthesis of Euglena gracilis and the diatom Phaeodactylum tricornum was more sensitive to UV-B inhibition than that of the dinoflagellates Heterocapsa triquetra and Prorocentrum minimum. Not only UV-B radiation but light had a photoinhibitory effect on photosynthesis in all four organisms. The effect on photosynthesis was observed both on the quantum yield and on the light saturation rate of photosynthesis