Carbon monoxide measurements are obtained from the analysis of the spectra provided by the Interferometric Monitor for Greenhouse Gases (IMG) instrument, which flew on board the Japanese ADEOS satellite. The averaging kernel function of the instrument, which provides the sensitivity of retrieved CO to the vertical atmospheric layers, is calculated. A sequential assimilation approach is used to incorporate this CO data set, along with a detailed associated error budget, into a global three-dimensional chemistry-transport model (MOZART version 2). We show how data assimilation allows one to highlight the differences between modeled and observed CO global distribution. Surface CO mixing ratios computed after assimilation of total columns provided by the IMG instrument are compared with the National Oceanic and Atmospheric Administration (NOAA)-Climate Monitoring and Diagnostics Laboratory (CMDL) in situ measurements, and a good agreement is found between the two data sets. ¿ 2001 American Geophysical Union