We examined the influence of solar ultraviolet-B radiation (UV-B; 280–315 nm) on the growth of Colobanthus quitensis plants by placing them under contrasting UV-B filters at Palmer Station, along the Antarctic Peninsula. The filters reduced diurnal biologically effective UV-B (UV-BBE) either by 83% (‘reduced UV-B’) or by 12% (‘near-ambient UV-B’) over the 63 day experiment (7 November 1998–8 January 1999). Ozone column depletion averaged 17% during the experiment. Relative growth and net assimilation rates of plants exposed to near-ambient UV-B were 30 and 20% lower, respectively, than those of plants exposed to reduced UV-B. The former plants produced 29% less total biomass, as a result of containing 54% less aboveground biomass. These reductions in aboveground biomass were mainly the result of a 45% reduction in shoot biomass, and a 31% reduction in reproductive biomass. Reductions in shoot biomass were owing to an 18% reduction in branch production by main shoots, while reductions in reproductive biomass were the result of a 19% reduction in individual capsule mass. Total plant leaf area was reduced by 19% under near-ambient UV-B, although total leaf biomass was unaffected because leaves had a greater specific leaf mass. The reduction in plant leaf area under near-ambient UV-B was attributable to: (1) production of 11% fewer leaves per main shoot system and plant, which resulted from an 18% reduction in branch production by main shoots. Leaf production per individual main shoot or branch was not affected; (2) shorter leaf longevity—main shoots contained 14% fewer green leaves at a given time; and (3) smaller individual leaves—leaf elongation rates were 14% slower and mature leaves were 13% shorter.