Ozone (03) phytotoxicity is driven by the concentrations the plants are exposed to, and also by meteorological and physiological variables. Thus, 03 uptake might be a better indicator of the phytotoxicity of this pollutant than 03 exposure. An ozone biomonitoring system based on the biomass ratio between an ozone resistant (NC-R) and an ozone-sensitive (NC-S) biotype of white clover (Trifolium repens cv. Regal) has been tested for 4 yr at four Spanish sites experiencing wide differences in their climatic and ozone exposure profiles. A complementary experiment was also carried out in open-top chambers exposing plants of both biotypes to three 03 treatments. Ozone exposure, air temperature, relative humidity and leaf stomatal conductance to water vapour were measured during the life span of both clover clones. These measurements enabled the total 03 accumulated dose (CUMF03) for each biotype to be calculated. The biomass ratio of both clones was best correlated with indices based on 03 uptake, rather than with indices related to 03 exposure. In fact, the best performance of the system was achieved when 03 uptake > 40 nl l-1 (CUMF03 40) was considered. The results support the need to relate 03 phytotoxicity to actual plant 03 uptake.