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Emiliania huxleyi (Lohm.) Hay et Miller is an important component of the phytoplankton in open ocean waters. The sensitivity of this cosmopolitan alga to natural levels of UVB radiation has never been tested. Since DNA is believed to be a major target of natural UVB radiation (UVBR: 280–315 nm) in living cells, experiments with E. huxleyi were performed using growth rate reduction and DNA damage as indicators of UVBR stress. Specific growth rate, cell volume, pigment content, and CPD (cyclobutane pyrimidine dimer) formation (a measure for DNA damage) were followed during and after prolonged exposure of a series of cultures to a range of UVBR levels. E. huxleyi was found to be very sensitive to UVBR: at a daily weighted UVBR dose of only 400 J·m⁻² ·d⁻¹ (BED_DNA300nm), growth was halted. At this UVBR level, both cell volume and contents of the major photosynthetic and photoprotective pigments …