The effects of excessive UV radiation on photosynthesis were investigated in the fresh water alga Cosmarium cucumis and in the marine alga Cryptomonas maculata.  The photosynthetic yield as well as the oxygen production were determined after exposure to the output of a strong UV source (UV-B: 7 W m^-2, UV-A: 7 W m^-2, PAR: about 5 W m^-2).  The effects of different wavelength ranges were determined by means of cut-off filters (WG 295 nm, WG 320 nm or WG 395 nm).  It was shown that Cosmarium cucumis is considerably less sensitive to high UV radiation compared to Cryptomonas maculata.  This is most likely due to the fact that this alga is adapted to high solar radiation in its natural habitat while Cryptomonas maculata is exposed to low light conditions in regions deeper in the water column.