Numerous investigators have demonstrated marked interspecific differences in tolerance of Antarctic marine phytoplankton to UVB exposure. Consequent changes in species composition have been proposed but as yet not demonstrated. We conducted competition experiments in which mixed cultures of Antarctic marine diatoms and colonial and flagellate stages in the life cycle of the haptophyte Phaeocystis antarctica were exposed to natural Antarctic solar irradiance. Results demonstrated UVB-induced changes in species cornposltion favouring the colonial stage of P. antarct~ca following 2 d exposure or less. These data indicate the potential for altered trophodynamlcs and carbon flux in Antarctic waters as a result of ozone depletion. Our results also show the limited predictive value of results obtained using UVB exposures above those likely to be experienced in the natural environment.