Tropospheric ozone (O3) is a strong oxidant, and is known to have serious negative effects on forest health. Lichens have bccn used as biomonitors of the effects of air pollution on forest health for sulfur and nitrogen pollutants. However, effects of O3 on lichens are not well understood, as past fumigation studies and community analyses produced conflicting results. We fumigated the lichen Ramalina menziesii Tay!. with three levels of O3 (10, 60, and 120 ppb) in two-month long experiments, testing photosynthesis, respiration, chlorophyll content, pigment degradation, and cell membrane leakage in response to the treatments, In southern California, the high ozone pollution season coincides with the wann arid summers. Many of the O3 fumigations conducted by others were located in climates with regular rainfall events. In our fumigations, we allowed the lichens to maintain ambient water status. We found that R. menziesii showed no signs of physiological decline in response to low and moderate levels of O3 fumigations and an insignificant response in some parameters to 120 ppb fumigations. Several plant species have been shown to be good indicators of O3 pollution, while community studies of lichens and ozone have shown that lichens are not. In our estimation, the lichen R. menziesii is not a good indicator species for O3 pollution, possibly due to antioxidant activity protecting the plant portion of the thallus (the algal cells within the symbiosis).