The effects of mycorrhizal status, phosphorus supply and CO2, partial pressure on production and allocation of biomass in seedlings from two populations of Pinus taeda L. were examined. Seedlings from a North Carolina and a Florida population were grown in sterile soil in a full-factorial experiment with one of two phosphorus treatments (low P, high P) and at one of two CO2 partial pressures (35.5, 71.0 Pa). One half of the seedlings were inoculated with Pisolithus tinctorius (Pers.) Coker & Couch hyphae and spores. Seedlings were harvested 60, 90 and 120 d after emergence. Elevated CO2 significantly increased total seedling dry mass in all treatments at all three harvests. Phosphorus limitation reduced seedling growth, and mycorrhizas increased seedling growth in seedlings limited by phosphorus supply. Generally, however, there were no interactions between CO2, phosphorus supply and mycorrhizal status on dry mass of seedlings. Mycorrhizas probably did not affect the response of dry mass to elevated CO2, because phosphorus limitation did not reduce response of dry mass to elevated CO2, Phosphorus-limited seedlings responded to elevated CO2, as a result of increased phosphorus uptake, resulting from increased total root dry mass, and increased phosphorus use efficiency. Although mycorrhizal colonization did not affect the response of biomass to elevated CO2, it significantly reduced the response of needle area. As a result, specific leaf area (leaf area per unit plant biomass) was lower in mycorrhizal seedlings grown in elevated CO2 than in mycorrhizal seedlings grown in ambient CO2 Because there were no effects on relative growth rate or seedling dry mass, reductions in specific leaf area suggest that elevated CO2 reduced the relative cost of the symbiosis.