ABSTRACT-The effect of solar radiation on photosynthesis, quantum yield of fluorescence and pigmentation under solar radiation in a laboratory system and in the natural habitat during a daily cycle was investigated in the red eulittoral alga Porphyra leucost~cta. Optimal (F,/F,,,) and effective (hF/Fm') quantum y~e l d of fluorescence, photosynthetic efficiency and maximal photosynthesis were drastically reduced by (!V-B (280-315 nm) radiation in algac transferred from the laboratory (grown in white light) to solar radiation for 3 h. The absorptance due to chlorophyll and biliproteins (phycoerythrin and phycocyanln) increased after 3 h of incubation In photosynthetically actlve radiation (PAR, 400-700 nm) and PAR+UV-A (315-400 nni) whereas no changes were observed under PAR+UV-A+UV-B In the field, the effcctive quanturn y e l d and oxygen production decreastd at noon durlng a diiily cycle, indicating photolnhibition. When solar UV-A and UV-B radiation was removed by means of selective filters, the average effcctive quantum yield increased by 28 -c 2 " . I SD. Chlorophyll and biliprotein concentration presented a daily pattern which was affected by UV radiation Both chlorophyll and biliprotein contents were reduced at noon, follo\~ved by recovery in the afternoon, but only when UV radiation was cut off. The difference between the maximal and minimal chl a concentration during the dally cycle was greater in the presence of UV radiation, but the difference for phycoerythrin and phycocyanin was greater under PAR alone. These results seem to indicate that the accuniulation of pigments is affected by solar radiation, with short-term changes induced by UV light. The ecological relevance of the daily variations of photosynthesis and pigmentation under solar radiat~on is discussed.