The influence of enhanced ultraviolet-B radiation (UV-B, 280-320nm) on competitive balance in wheat (Triticum aestivum L.) and wild oat (Avena fatua L.) was examined in the field for each of 6 years using replacement series experiments. The relative competitive status of wheat increased significantly under UV-B enhancement in two of the years, when precipitation was relatively high, and over all years when data from all years were combined. Shifts in competitive balance occurred in response to UV-B enhancement despite no significant changes in total shoot biomass in either monocultures or mixtures. Associated with shifts in competitive balance under UV-B treatment in the field and in a glasshouse competition experiment were similar, differential effects of UV-B on species growth form: leaf insertion heights and leaf blade lengths were reduced to a greater extent in wild oat than in wheat in mixtures exposed to enhanced UV-B. These changes in individual plant morphology were related to shifts in foliage height distribution in mixed canopies with wheat overtopping wild oat to a greater degree under UV-B treatment.