In this preliminary study the flagellated, planktonic green alga Chlamydomonas reinhardtii was exposed to natural UV doses in an oligotrophic alpine lake and a mesotrophic lowland lake, both situated in Norway. The algae, which were incubated in glass, quartz or mylar-covered flasks at different depths, showed a pronounced loss of flagella with increasing UV radiation. Apparently, UV-A was as detrimental as UV-B for the flagella. The potential role of C. reinhardtii as a bio-dosimeter for UV stress, and potential food chain effects are discussed.