The flagellated, planktonic green alga Chlamydomonas reinhardtii was exposed to UV-B doses (peak intensity at 312 nm) from 0.6 to 36 kJ m-2. Flagellar status, uptake of 33P and growth rates were judged in both P-limited and P-saturated algae. A linear relationship was found between loss or withdrawal of flagellae and UV exposure. Even 10 min (0.6 kJ m-2 exposure initiated inactivation of flagellae and total loss as well as signs of cell wall destruction were found at the highest doses Within 72 h after irradiation, flagellar recovery had occurred in 34% of non-limited and 87% of P-limited cells. Uptake of P was apparently enhanced at low doses, but fell to almost zero at the higher doses. In P-saturated cells, an initial high uptake was followed by a pronounced P leakage upon radiation. A similar pattern was found for growth rates, where notably P-limited cells gained increased growth at low doses, while growth was drastically reduced at 16 kJ m-2. The data show that major functional parameters in this freshwater flagellate are affected by moderate doses of UV-B, and that reduced P uptake is well correlated with inactivation of flagellae and growth inhibition.