A source analysis of carbon monoxide (CO) over the Indian Ocean is presented using marked tracers in a chemistry general circulation model. The model includes a nonmethane hydrocarbon chemistry scheme and has been used at two different resolutions (3.75¿¿3.75¿ and 1.9¿¿1.9¿). European Center for Medium-Range Weather Forecasts meteorological analyses have been assimilated into the model to represent actual meteorology during February and March of 1999. A comparison with measurements indicates that the model simulates realistic CO distributions. In general, the model performance is more realistic at higher resolution. Discrepancies exist close to the Indian coasts, possibly related to a sea breeze circulation at the Indian west coast, not resolved by the model. Discrepancies are also found in the vicinity of convection at the Intertropical Convergence Zone (ITCZ). The marked tracer study suggests that biofuel use and agricultural waste burning in India are major CO sources for the Indian Ocean north of the ITCZ, with minor contributions from Middle East, China, and Southeast Asia. In the much cleaner boundary layer over the southern Indian Ocean, CO from hydrocarbon oxidation is a dominant source. There are no other regions around the globe where biofuel use and biomass burning contribute so much to the CO mixing ratios. In general, most of the Asian CO over the Indian Ocean remains north of the ITCZ, although some of the CO is transported to the southern hemisphere in the free troposphere near the African east coast. ¿ 2001 American Geophysical Union