Exposure to ultraviolet radiation may be an important factor controlling the upper limit of distribution for benthic marine algae. Growth and photosynthesis of sublittoral macroalgae have previously been shown to have a high sensitivity to UV. This work examines the photosynthetic response of the brown algae Laminaria digitata to range of UV-A and UV-B wavelengths, obtained by combining fluorescent sunlamps with cut-off filters, in an attempt to find a function which best describes the wavelength-dependence of the response. The maximum rate of photosynthetic oxygen evolution was reduced by al1 UV treatments. The biologically-effective weighted dose given in each experiment was calculated for a range of commonly-used ultraviolet action spectra. The reduction in photosynthesis was best explained by weighting the UV dose with the chloroplast photoinhibition action spectrum of Jones and Kok (1966), the effectiveness of which extends through the UV-B and UV-A regions. Action spectra which only gave weight to the UV-B region of the spectrum (DNA damage, general plant damage) did not account for the photosynthetic reduction observed after UV-A-only treatments. Therefore, it is to be expected that both the UV-A and UV-B components of natural solar irradiance will be important in causing depression of photosynthetic rates for this species.