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)} 0.70–3.7, 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_{(290–315 nm)} 2.6–40.7, UV-B_{(290–320 nm)} 6.7–78.5, UV-A_{(315–400 nm)} 228–1539, UV-A_{(320–400 nm)} 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, F_v/F_m) 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 UV 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.