Marine primary production has been estimated on the basis of photosynthetically active radiation (PAR,400-700 nm),while effects of UV radiation (280-400 nm) has either been neglected or regarded inhibitive.We have shown now that UVR could act as an additional source of energy for marine photosynthesis by phytoplankton in the absence of PAR or under cloudy weather conditions.When PAR of sunlight was completely filtered out,photosynthetic carbon fixation by the phytoplankton assemblages from surface seawater increased linearly with increased irradiance of UVR,with the maximal rate ranged 0.55μg C (μg chl a) -1 h -1 at 33.3 W m -2 (54% of the noontime level) for 280-400 nm to 0.8 μg C (μg chl a) -1 h -1 at 30 W m -2 for 305-400 nm UVR.