Terrestrial responses to increasing atmospheric CO2 are important to the global carbon budget. Increased plant production under elevated CO2 is expected to increase soil C which may induce N limitations. The objectives of this study were to determine the effects of increased CO2 on 1) the amount of carbon and nitrogen stored in soil organic matter and microbial biomass and 2) soil microbial activity. A tallgrass prairie ecosystem was exposed to ambient and twice-ambient CO2 concentrations in open-top chambers in the field from 1989 to 1992 and compared to unchambered ambient CO2 during the entire growing season. During 1990 and 1991, N fertilizer was included as a treatment. The soil microbial response to CO2 was measured during 1991 and 1992. Soil organic C and N were not significantly affected by enriched atmospheric CO2. The response of microbial biomass to CO2 enrichment was dependent upon soil water conditions. In 1991, a dry year, CO2 enrichment significantly increased microbial biomass C and N. In 1992, a wet year, microbial biomass C and N were unaffected by the CO2 treatments. Added N increased microbial C and N under CO2 enrichment. Microbial activity was consistently greater under CO2 enrichment because of better soil water conditions. Added N stimulated microbial activity under CO2 enrichment. Increased microbial N with CO2 enrichment may indicate plant production could be limited by N availability. The soil system also could compensate for the limited N by increasing the labile pool to support increased plant production with elevated atmospheric CO2. Longer-term studies are needed to determine how tallgrass prairie will respond to increased C input.