We have characterized the photosynthetic response to ultraviolet radiation (UV) of natural phytoplankton assemblages in Antarctic (Southern Ocean) waters. Biological weighting functions (BWFs) and exposure response curves for inhibition of photosynthesis by UV were measured during spring-time ozone depletion (October-November). Two different models were developed to relate photosynthesis to UV exposure. A model that is a function of the duration of exposure (BWF H) applied to assemblages in the well-mixed open waters of the Weddell-Scotia Confluence (WSC, 60° S, 50° W), since responses were a function of cumulative exposure and recovery rates were slow. These assemblages had a variable but generally high sensitivity to UV. A steady-state model (BWF E) applied in the shallow waters near the Antarctic Peninsula (Palmer Station, 64° S, 64° W), where inhibition was a function of irradiance (reciprocity failed), and recovery was rapid. Using information on the time-dependence of photosynthesis in assemblages with active repair, inferences were drawn on the relative contribution of damage and recovery processes to the UV weights. BWFs for Palmer phytoplankton sampled during periods of pack-ice cover had both higher damage and higher repair than BWFs for WSC assemblages. BWFs for Palmer phytoplankton sampled during open water periods had about the same damage weights as Weddell-Scotia assemblages but had a higher repair rate. Solar exposures of more than 10 min were predicted to have generally less effect on Palmer phytoplankton than the WSC phytoplankton