A continuing decline in stratospheric ozone concentrations caused by atmospheric pollutants has stimulated interest in consequences of increased penetration of ultraviolet-B radiation to the surface of the earth. The authors examined the effect of supplemental UV-B (290-320 nm) on photosynthetic characteristics of different aged needles of 3-year-old, field-grown loblolly pine (Pinus taeda L.). Needles in four age classes were examined: (1) most recently fully expanded, year 3; (2) first flush, year 3; (3) final flush, year 2; and (4) oldest needles still present, year 2. Enhanced UV-B radiation caused a statistically significant decrease (6%) in the ratio of variable to maximum fluorescence (F sub v)/(F sub m) following dark adaption only in needles from the youngest age class, suggesting transient damage to photosynthesis. Exposure to enhanced UV-B radiation caused a 20% decrease in total biomass and a 4% decrease in needle length of needles in age classes 1, 2, and 4, respectivly.