The combined effects of increased UV-B and temperature on natural marine phytoplankton from the St. Lawrence Estuary (Canada) were examined in an 8 d mesocosm experiment carried out in Rimouski (Québec, Canada) in August 2008. We tested the hypothesis that increased temperature (+ 3°C) will offset algal growth suppression by UV-B (78% UV-B increase) using duplicate mesocosm experiments containing natural phytoplankton assemblages. The response of the entire phytoplankton community, in terms of HPLC pigment-based phytoplankton bio mass, community composition (CHEMTAX), xanthophyll cycles photoprotection and quantum yield of photosystem II (the ratio of variable to maximum fluorescence: F v /F m ), showed a significant influence of temperature (negative on small phytoplanktonic cells, < 5 µm, and positive on larger diatoms) but only after the peak of the diatom bloom, when nutrients became limited. Interactions between temperature and UV-B treatments were significant only for small cells during post-bloom; UV-B induced an increase in phytoplankton biomass at the normal temperature but had no effect at warmer temperatures. Enhancing UV-B delayed the bloom slightly under the normal temperature and spread it over a longer period of time, with no sign of major cellular damage. Our re sults do not support the tested hypothesis, and they suggest that temperature plays a greater role than UV-B radiation in structuring phytoplankton communities, possibly favouring diatoms rather than small cells in a warmer climate scenario. Other effects such as grazing or coastal eutrophication should be considered in future studies.