Technical Abstract: Tropospheric O3 reduces growth and yield of many crop species, whereas CO2 ameliorates the negative effects of O3. Thus in a combined elevated CO2 and O3 atmosphere, seed yield is at least restored to that of charcoal-filtered (CF) air at ambient CO2. The CO2-induced yield increase in CF air is highly variable, suggesting other potential resource limitations. To understand such variability in response, we tested that (1) competition for resources precludes some of the CO2 enhancement on biomass and yield; and (2) O3 reduces competition in elevated CO2. We grew rice (Oryza sativa L.) at 5 densities in CF and O3-fumigated (+O3) air at ambient (A) and elevated [CO2] (+CO2) in 1997 and 1998. O3 reduced biomass by 25% and seed yield by 13-20% in A, but had little effect in +CO2. A competition model of biomass and yield response to density based on resource availability without competition showed that fewer resources were used for biomass in +O3 than in CF (avg. 53 vs. 70%) in A, while in +CO2 85% of resources were used for biomass regardless of O3 suggesting greater depletion of resources. The enhanced biomass response to CO2 with O3 is consistent with a 22% greater CO2 enhancement ratio (mass in +CO2 air/mass in A air; ER) in +O3 than in CF air. For seed yield, few resources were used (avg. 17% and 25% for CF in 1997 and 1998, respectively), and ER was 13% greater in +O3. With competition the rate of change of individual plant biomass to density was not affected by +CO2 in CF air in 1997 but was increased 19% with more nutrients in 1998, indicating resource limitations with +CO2. The rate of change of individual plant yield to density was reduced with CO2 in 1997 and unchanged in 1998 showing a different response to resource limitation for reproductive biomass. The resource use in +O3-A suggested that increased density and soil fertility might compensate for pollutant damage. Although ambient [O3] can modulate the response to elevated CO2, resource limitation precludes the CO2 fertilization impact and both factors need consideration for better management and forecasts of future productivity.