Marine microalgae exposed to ultraviolet radiation (UV) have complex adaptive responses provided by a series of protection and repair mechanisms. Interspecific differences in UV sensibility could result in differential selection of the more tolerant species, having consequences for the structure of phytoplankton assemblages. The relative importance of protection and photorepair mechanisms of microalgal cells exposed to potential UV-B stress was studied in monocultures with different taxonomic, ecological and size characteristics obtained from the Chilean coast. Differences in photosynthesis and growth rates were predicted, since the ability to effectively acclimate to UV is not universal between microalgal species. The dinoflagellate Alexandrium catenella Whedon et Kofoid Balech, the diatom Phaeodactylum tricornutum Bohlin, the chrysophyte Aureococcus sp. and the cyanobacterium Spirulina subsalsa Oersted were acclimated during exponential cell growth under PAR + UV-A radiation (365 nm, 140-240 kJ m-2 d-1) and thereafter exposed 2 h d-1 to high and low UV-B radiation (312 nm, maximum 3.1 kJ m-2 d-1) at the center of the 16 h light period. Measured parameters were growth rates (µ), in vivo spectral absorption, cellular fluorescence capacity, pigment concentration, photosynthesis and photoreactivation during three cycles in controls and treatment samples. Growth rates diminished less than 35 % in Phaeodactylum and Aureococcus compared to 80-100 % decrease in Alexandrium and Spirulina. In these two last species, a significant increase in UV absorbing substances was observed, probably related to the presence of mycosporine-like aminoacids (MAAs) and scytonemin, respectively, and also lower photoreactivation efficiency compared to Phaeodactylum and Aureococcus. The analysis of photosynthetic performance under different PAR/UV-A ratios for Alexandrium and Phaeodactylum, could also explain the differences in µ. These results suggest that in time, species with high rates of photorepair might be more tolerant to UV-B than those species, which depend on the synthesis of UV absorbing compounds as their principal protection mechanism