The effect of ozone on tree growth and metabolism has been studied widely. Despite the research emphasis, relatively little is known about how the below-ground component responds when shoots are exposed to ozone, even though evidence suggests that ozone can affect roots more than shoots. Undemanding how ozone affects carbohydrate allocation throughout the plant is essential to understanding the mechanisms of response to ozone. The purpose of this study was to follow the allocation and metabolism of carbon in a Pinus Ponderosa Laws.-Hebeloma crustuliniforme (Bull.: St. Amans) Quel seedling system under ozone stress. The hypothesis that ozone affects carbon transport below ground and overall sink strength of roots. similarly in mycorrhizal and non-mycorrhizal seedlings was tested. To test the hypothesis, a unique culturing system was used to quantify carbon movement to all components of the symbiosis and to construct an overall budget for carbon for both mycorrhizal and non-mycorrhizal seedlings. Fluxes of CO2 and carbon allocation were followed by measuring instantaneous CO2 flux and by 14C labelling. Two experiments were conducted that differed in their total ozone exposure (39.3 ppm h in expt 1, and 58.1 ppm h in expt 2). Mycorrhizal inoculation significantly increased CO., assimilation rates (A) and A/R (R = shoot respiration) ratios in both experiments compared with non-mycorrhizal seedlings. Ozone exposure in expt 2 significantly decreased the A/R ratio (P < 0.003) in both mycorrhizal treatments. Below-ground respiration was significantly greater in mycorrhizal than in non-mycorrhizal seedlings in both experiments, and was not affected by ozone exposure, Intact, extramatrical hyphal respiration was lower by 33% in seedlings exposed to ozone, but differences were not statistically significant (P ≤ (0.167). Mycorrhizal seedling roots reached maximum respiratory 14CO2 release rates c. 5 h and < 20 h earlier than non-mycorrhizal seedlings in expts 1 and 2, respectively, suggesting accelerated transport of 14C below ground in mycorrhizal seedlings. Mycorrhizal seedlings also exhibited greater rates of 14C release below ground than non-mycorrhizal controls. The maximum rate of respiratory release of 14CO2 below ground was significantly reduced by exposure to ozone in both mycorrhizal and non-mycorrhizal treatments. Ozone significantly reduced 14C activity in the fungus of mycorrhizal plants. This constitutes the first report of an ozone-induced reduction in carbon allocation to the fungal symbiont in a mycorrhizal association. The results suggest a substantial impact of ozone on the carbon balance of the mycorrhiza: however, there was no evidence to suggest that mycorrhizal and non-mycorrhizal ponderosa pine seedlings responded differently to ozone stress.