In this study the effect of solar UV radiation on thephysiology of the intertidal green macroalga Ulva lactuca L.was investigated. A natural Ulva-community at the shoreof Helgoland was covered with screening foils, excludingUV-B or UV-B + UV-A from the solar spectrum. In the sampledmaterial, changes in the activity and concentration ofRubisCO, and the concentration of photosynthetic andxanthophyll cycle pigments were determined. Exclusionof UV-radiation from the natural solar spectrum resultedin an elevated overall activity of RubisCO, related to anincrease in its cellular concentration. Among the photosyntheticpigments, lutein concentration was substantially elevatedunder UV exclusion. In addition, marked UV effects on thexanthophyll cycle were found: exclusion of solar UV-radiation(and particularly UV-B)resulted in an increased ratio ofzeaxanthin concentration to the total xanthophyll content,indicating adverse effects of UV-B on the efficiency ofphotoprotection under high irradiances of PAR. The resultsconfirm a marked impact of present UV-B levels on macroalgalphysiology under field conditions.