Soybeans were grown outdoors in field plots during 1983 and 1984 under ambient and 2 supplemental levels of biologically effective ultraviolet-B (UV-B) radiation (daily supplemental dose: 3.0 or 5.1 effective. These correspond to a 16 and 25% ozone reduction during clear sky conditions on the summer solstice. In 1983, neither cultivar showed any significant differences in growth, CO2 assimilation, transpiration, stomatal conductance nor leaf water potential under enhanced levels of UV-B irradiation. By contrast in 1984, during late vegetative and early reproductive stages of development, leaf area and CO2 assimilation in Essex were significantly reduced while stomatal conductance was increased by UV-B irradiation. The study illustrates that the effectiveness of enhanced levels of UV-B radiation is related to several factors, including microclimatic conditions prevailing during the growing season, cultivar, and the stage of plant development.