Fires in the northern interior have long been known to smolder for long periods. Because emission rates from smoldering smoke are small in comparison to rates of emissions during flaming, however, and because it is difficult to monitor smoldering fires, there are few observations of smoldering smoke beyond a couple of hours after ignition. At the 1999 Frostfire prescribed-fire experiment in interior Alaska a network of carbon monoxide dosimeters were placed at automated ground observing stations. The 55-day sample period measured hourly-averaged CO concentrations in the valley of over 5 ppm each night. At least one active period occurred 28 days after ignition when hourly-averaged CO reached well over 10 ppm and elevated CO concentrations were observed at surrounding ridgetops. These values are within ranges measured for firefighter personnel at active fires and close to recommended thresholds for air quality standards. Further research is needed to determine if such levels may contribute to the global carbon budget more significantly than previously realized.