In most terrestrial ecosystems, the amount of substrate entering the soil from plant litter production is only sufficient to meet the maintenance requirements of soil microorganisms, allowing for no net annual growth. However, the rising atmospheric CO2 concentration has the potential to alter such a balance by increasing plant litter production, and hence the amount of substrate available for heterotrophic metabolism in soil. In a recent experiment, we observed that greater belowground plant litter production at elevated atmospheric CO2 significantly increased the biomass of soil microorganisms in both rhizosphere and non-rhizosphere soil. Because soil microorganisms differ in their ability to convert substrate into biomass, we hypothesized that greater plant litter production at elevated CO2 should shift community composition as fungal populations increase in response to greater substrate availability. We used a molecular technique, phospholipid fatty acid (PLFA) analysis, to gain insight into the composition of soil microbial communities beneath Populus grandidentata growing at ambient and twice-ambient atmospheric CO2. PFLAs extracted from rhizosphere and non-rhizosphere soil were derivatized and identified using gas chromatography and mass spectrometry. After one growing season the proportions of bacterial, actinomycetal, and fungal PLFAs were not significantly influenced by elevated atmospheric CO2 in either rhizosphere or non-rhizosphere soil. However, clear differences were present between microbial communities in rhizosphere and non-rhizosphere soil. Although enhanced belowground plant litter prroduction under elevated atmospheric CO2 increased the biomass of soil microorganisms, we have no evidence to suggest that such an increase occurred through a shift in community composition, at least in the short term.