The interactive effects of ozone (O3) and water stress on alfalfa were studied to determine how water stress could alter these functions, and to describe the cumulative effects of multiple-year exposures to O3 on this perennial crop. Water stress reduced total seasonal yield about 10% in 1984 and 27% in 1985. Ozone significantly reduced yields in both years, and the interaction between O3 and water stress was statistically significant in 1985 and for combined 1984 and 1985 years. Ozone dose-alfalfa yield loss functions were homogeneous between 1984 and 1985 and no evidence of a cumulative effect of multiple-year exposure to O3 was found on top growth. However, crown (underground stem) weights were significantly reduced by O3, suggesting that continued exposure to O3 could shorten the productive life of alfalfa stands, in addition to its effect on yield. (Copyright (c) 1988 ASA, CSSA, SSSA.)