Daily variations of photosynthetic performance of the brown alga Dictyota dichotoma (Hudson) Lamouroux (Dictyotales) (estimated by Fv/Fm ratio and net O2-based light saturated photosynthesis) were determined under full-spectrum solar radiation (PAR+UVA+UVB), solar radiation depleted of ultraviolet-B (PAR+UVA) and solar radiation depleted of total ultraviolet (PAR). In the daily course the photosynthetic response of the alga was inhibited by the three solar radiation conditions at midday, but it recovered during the afternoon to those values measured in early morning only under PAR+UVA+UVB, or PAR alone. Under solar radiation depleted of UVB, the recovery of photosynthesis was significantly lower than in the other two solar radiation conditions.The relative electron transport rate (ETR) as a function of PAR were calculated for the three radiation treatments, at noon after 2 h exposure. The ETR was strongly inhibited under these conditions. In the PAR and PAR+UVA+UVB treatments, photosynthesis recovered after 1.5 h in dim irradiance (<10 µmol photons m-2 s-1) and obtained values measured in early morning. In the absence of UVB, the inhibitory effect was persistent, visible in a low initial slope and lower ETR at higher levels of irradiance. These data suggest that UVB may be involved both in the impairment and the recovery of photosynthesis of D. dichotoma.