Soybean plants(Glycine max [L.] merr.) inoculated with Bradyrhizobium japonicum MN110 were grown in growth chambers under 400 or 800 ㎕·ℓ^(-l) atmosphtric CO₂ and harvested at 25, 28, 32, and 35 DAT to examine the effect of CO₂ enrichment on phosphorus accumulation, uptake, and utilization efficiency during vegetative growth. Phosphorus concentration in leaf was lower in high CO₂ plant by 47% at 25 DAT and 34% at 35 DAT than those in the control plant but phosphorus concentrations in stem, rood and nodule were not affected by CO₂ enrichment. Total phosphorus accumulation increased 3.9-fold in high CO₂ plant and 3.2-fold in the control plant between 25 and 35 DAT. Elevated CO₂ caused a decrease in the whole plant phosphorus concentration by 35%, which was due almost entirely to a decrease in the phosphorus concentration of leaves. CO₂ enrichment increased phosphorus utilization efficiency in the whole plant by 70% during the experimental period. Plants exposed to high CO₂ had larger root systems than under ambient CO₂, but high CO₂ plants had lower P-uptake efficiency. Averaged over four harvests, plants at high CO₂ had 38% larger root mass that was more than offset the 20% lower efficiency of P-uptake and accounted for increased phosphorus accumulation by high CO₂ plant. These results suggest that the reduced phosphorus concentration in soybean plant under CO₂ enrichment may be an acclimation response to high CO₂ concentration or enhanced starch accumulation, resulting in the plants to have a lower phosphorus requirement on a unit dry weight basis or a high phosphorus utilization efficiency under these conditions.