Surface CO variations from continuous measurements during 1997–2000 at a tropical site in rural Thailand are reported for the first time. The CO seasonal cycle at Srinakarin (14°22′N, 99°07′E, 296 m above sea level) shows a maximum during the late dry season (February–March) and a minimum during the midwet season (June–August) with a larger amplitude than other remote sites in the Northern Hemisphere. The 4-year annual average, averaged monthly minimum, and averaged monthly maximum of CO mixing ratios are 260, 97, and 560 ppbv, respectively. The long-range transport of air masses driven by the Asian monsoon and biomass burning activity in continental Southeast Asia are found to be the main factors controlling CO variation in Thailand. Trajectory analysis indicates that during the wet season when the southwest monsoon from the Indian Ocean prevails, relatively low CO mixing ratios of 101 ± 32 ppbv on average are observed. Meanwhile, an average of 306 ± 83 ppbv of CO is observed when the site is influenced by regionally polluted continental air transported by the northeast monsoon from East Asia during the early dry season. Air mass transport, however, cannot explain high CO mixing ratios during the late dry season. Analysis of fire hot spots from the Along Track Scanning Radiometer (ATSR) satellite data indicates that the seasonal cycle of biomass burning in continental Southeast Asia is regular and is at a maximum in March, coincident with the CO maximum. The good correlation between CO and the number of ATSR hot spots provides evidence that this high CO during the late dry season is due to biomass burning in continental Southeast Asia. In addition, we found possible influences from the 1997 Indonesian fires and the 1998 Siberian Fires on the CO variations at Srinakarin.