The study was conducted to determine the physiological and growth responses of cotton (Gossypium hirsutum L.) to the interaction of ozone (O3) and drought stress. Cotton (cv SJ-2) was grown in open-top chambers in the field at three levels of soil water and exposed to charcoal-filtered air, nonfiltered air (NF), and NF x 1.25, and NF x 1.5 ambient O3 concentrations in Riverside, CA from June to October 1986. Ozone reduced carbon fixation an average of 74.6% in optimally watered (OW) plots, 63.4% in suboptimal (SO) plots, but only 19.3% in severely water-stressed (SS) plots. Leaf and stem biomass in OW and SO plots showed similar linear reductions in mass in response to increased O3 concentrations, but SS plots showed no response to O3 except at the highest O3 treatment (seasonal 12-h O3 mean of O-111 ppm 218 microgm(sup -3)). These results showed that moderately water-stressed cotton had similar physiological and growth responses to O3 as well-watered plants, but severely water-stressed cotton showed little response to O3 at ambient O3 concentrations. (Copyright (c) 1988 Elsevier Applied Science Publishers Ltd.)