This study reports 5 years of (1998–2003) data on continuous solar-irradiation measurements from a scanning spectroradiometer (SUV-100) in Valdivia, Chile (39° S), accompanied by evaluation of the impact of ultraviolet radiation (UVR) on marine macroalgae of this site. UVR conditions showed a strong seasonal variation, which was less pronounced toward longer wavelengths. Daily maximum dose rates (clear days) averaged in winter-summer: UV-B(290–315 nm) 0.30–2.1, UV-B(290–320 nm) 0.703.7, UV-A(315–400 nm) 20.6–62.1, UV-A(320–400 nm) 20.2–60.5 W m-2, and photosynthetically active radiation (PAR) 969–2423 μmol m-2 s-1. The corresponding daily doses (all the days) ranged: UV-B(290–315nm) 2.6–40.7, UV-B(290–320 nm) 6.7–78.5, UV-A(315–400 nm) 228–1539, UV-A(320–400nm) 224–1501, and PAR 2008–13308 kJ m-2 d-1. Taking into consideration action spectra of a biological interest, the risk of UV exposure could be up to 37 times higher in summer than in winter. The photosynthetic activity (as maximum quantum yield of chlorophyll fluorescence, Fv/Fm) of the brown alga Lessonia nigrescens from the infralittoral zone was markedly more sensitive to UVR than of the green alga Enteromorpha intestinalis from the upper midlittoral, and the UV-B wave band increased markedly photoinhibition. In L. nigrescens, maximal photoinhibition (40%) took place at weighted (the action spectrum for photoinhibition of photosynthesis) UVR doses of 800 kJ m-2, irrespective of the season (corresponding midsummer daily dose in Valdivia is 480 kJ m-2). In winter, when this alga was at its most sensitive, the weighted W dose causing 35–40% photoinhibition was around 200 kJ m-2. In E. intestinalis, weighted doses of 800 kJ m-2 resulted in low photoinhibition (<10%) and no clear seasonal patterns could be inferred. These results confirm that midday summer levels of UV-B and their daily doses in southern Chile are high enough to produce stress to intertidal macroalgae.