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 tricomution was more sensitive to UV‐B inhibition than that of the dinoflagellates Heterocapsa triquetra and Prorocentrum minimum. Not only UV‐B radiation but also high visible 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. The dinoflagellates, in contrast to E. gracilis and P. tricorntum, absorbed strongly in the UV region (334 nm) and their absorption peaks increased after growth under high visible light or with or without UV‐B radiation for one week. The swimming speed of H. triquetra decreased more after low visible light and UV‐B radiation compared to high visible light and UV‐B radiation. The negative effects of UV‐B radiation on P. minimum and E. gracilis were most pronounced after high visible light.