We analyze data from the National Oceanic and Atmospheric Administration/Climate Monitoring and Diagnostics Laboratory global air sampling network in order to extract the signatures of isotopic (13C/12C) discrimination by the terrestrial biota and of fossil fuel combustion for the regions surrounding the sampling sites. We utilize measurements of carbon monoxide (CO) to give an estimate of the contribution of fossil fuel combustion to the short-term variability of carbon dioxide (CO2). In general, variations of CO2 are more strongly dominated by biological exchange, so the isotopic signature of fossil fuel combustion, while consistent with inventory estimates, is not well constrained by the observations. Conversely, results for isotope discrimination by the terrestrial biosphere are not strongly dependent on assumptions about fossil fuel combustion. Our analysis appears valid primarily for stations fairly near continental source/sink regions, particularly for midlatitude regions of the northern hemisphere. For these stations we derive a mean discrimination of -16.8 per mil (?), with site-to-site variability of 0.8? (1 standard deviation) and with little or no consistent latitudinal gradient. ¿ 1998 American Geophysical Union