The influence of phosphorus fertilization and mycorrhizal development on net photosynthesis, dry matter, shoot-root partitioning, and shoot dark respiration was examined in loblolly pine. Seedlings, initially grown for 10 weeks under low phosphorus (P) conditions, were either fertilized with 0.1, 1.0, 4.0, 8.0, or 12.0 g g-1 p and left nonmycorrhizal, or inoculated with three different concentrations of a mycelial slurry of Pisolithus tinctorius and fertilized with only 0.1 g g-1 p. After an additional 7 weeks, net photosynthetic rate of all seedlings was measured followed by harvesting and determination of P content of foliage, and dry matter of foliage, stem, and roots. Both phosphorus fertilization and mycorrhizal development caused a substantial increase in the net rate of photosynthesis and dry matter accumulation, but neither treatment resulted in significant changes in shoot-root allocation of dry matter or shoot dark respiration. At low and medium levels of mycorrhizal development, the net photosynthetic response to mycorrhizal formation appeared to be due to enhanced P nutrition. However, at high levels of mycorrhizal development, as much as 17% of the total increase in the net photosynthetic rate that resulted from mycorrhizal development, may be attributed to mechanisms other than enhanced phosphorus nutrition. For. Sci. 36(1):101-112.