Individual and interactive effects of elevated atmospheric COT and soil N fertility on juvenile ponderosa pine(Piiius ponderosaDougl. ex Laws.) were examined, focusing primarily on root system development. Controlled-environment growth chambers permitted creation of atmospheres with 700 µL L-1, 525 µL L-1, or ambient CO2concentrations. Seedlings were reared from seed in soil with a total N concentration of 900 µL L-1or in soil amended with sufficient (NH4)2S04 to increase total N by 100 mgg. An initial whole-seedling harvest after 4 months and a final harvest after 11 months permitted assessment of shoot and root growth and ectomy-corrhizal colonization as affected by each combination of COi and N treatment. Throughout the study, shoot growth was moderately increased by the 525 µL L-1' CCb atmosphere, evident primarily in volume measurements, but not by 700 µL L-1CCb. The stimulation of shoot growth by the intermediate CCb concentration was confined to low soil N initially but to high N at the conclusion of the study. Total root system length and specific root length were increased by 525 µL L-1CCb after 4 months and specific root length again after 11 months. Both enriched atmospheres increased root volume, total root length, and fine root mass after 11 months as well. Elevated CO2 effects on shoot/root ratio were nonsignificant initially, but at the final harvest, reduced ratios prevailed in the 525 and 700 µL L-1treatments, a result accentuated by high soil N. Total numbers of mycorrhizae formed were increased by 525 µL L-1CCb throughout the study, and mycorrhizal count and the percentage of root length colonized were increased by 700 µL L-1CCb after II months. These results suggest that the growth of root organs is stimulated by CCb enrichment substantially more than that of shoot tissues in juvenile ponderosa pine.