We measured the concentration and isotope ratio of dissolved inorganic carbon (DIC) in groundwater associated with denitrification (Corg+NO3-=CO2+N2) in an agriculturally impacted site in southwestern Michigan. Samples with the lowest nitrate levels also had low dissolved oxygen content and were more depleted in Δ13C than background groundwater. All the samples had DIC concentrations in excess of titratable alkalinity. The magnitude of this DIC in excess of alkalinity correlated with a decreasing Δ13C attesting to the presence of CO2 derived from organic carbon. Carbon dioxide is one of the by-products of denitrification. Since the carbon isotope ratio of carbon dioxide produced during denitrification can be expected to be similar to that of the substrate consumed in the process, this can serve as a useful tool in monitoring denitrification. The estimated carbon isotope ratio of source carbon added to the DIC pool via denitrification was close to the value obtained for the organic carbon in the aquifer solids, suggesting that the carbon substrate used in denitrification came from soil organic matter. The results provide support for the hypothesis that the concentration and carbon isotope ratios (Δ13C) of dissolved inorganic carbon (DIC) in groundwater might be useful to monitor denitrification.¿ 1997 American Geophysical Union