A two-year study was conducted to evaluate the effects of O3 and SO2, both alone and in combination, on the yield and quality of a mixed crop of red clover and timothy. Sulfur dioxide had no effect on yield and there were no interactions between the pollutants. In the first year of the study, exposure to increasing concentrations of O3 led to reductions in yield primarily through reductions in the yield of red clover; there was little effect on the growth of timothy. In the second year, red clover had become less prevalent in the stand and O3 had no impact on total yield since the stand was dominated by the more O3-resistant timothy. In summary, the effect of O3 on the yield of the stand of mixed forage was not only a function of the concentration of the pollutant, but also of the relative O3-sensitivity and contribution to the stand for each species. Evaluations of crop quality indicated that with the exception of an increase in foliar sulfur, none of the treatments altered the variables assessed.