As atmospheric carbon dioxide (CO(2)) concentrations rise, one important mechanism by which plants can gain greater access to necessary soil nutrients is through greater investment in their mycorrhizal symbionts. In this study, we tested the hypotheses that (1) plants increase C allocation to ectomycorrhizal fungi (EMF) under elevated CO(2) conditions, (2) N fertilization decreases C allocation to EMF, and (3) EMF activity at the site of symbiotic C and nutrient exchange is enhanced with CO(2) enrichment. To test these hypotheses, we examined expression levels of Pinus taeda genes encoding monosaccharide transport (MST) and ammonium transport (AMT) proteins thought to be involved in symbiotic C and N movement, respectively, from mycorrhizal root tips exposed to CO(2) and N fertilization. We also examined EMF ribosomal RNA expression (18S rRNA) to determine EMF activity. There was a trend toward lower relative MST expression with increased CO(2). AMT expression levels showed no significant differences between control and treatment plots. EMF 18S rRNA expression was increased in CO(2)-enriched plots and there was a marginally significant positive interactive effect of CO(2) and N fertilization on expression (p = 0.09 and 0.10, respectively). These results are consistent with greater C allocation to EMF and greater EMF metabolic activity under elevated CO(2) conditions, although selective allocation of C to particular EMF species and greater fungal biomass on roots are plausible alternative hypotheses.