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Measurements of the 13C/12C and 18O/16O ratios and 14C of atmospheric CO were carried out for 2 years (1997 and 1998) at Happo (36°41′N, 137°48′E, 1840 m above sea level) in Japan. This is the first measurement of time series for isotopic compositions of CO at a remote site in Asia. The seasonal cycle of the 18O/16O ratio shows a maximum of 10‰ (Vienna SMOW) in February and a minimum of 2‰ in July. This minimum value is heavier than the values reported for high latitude in the Northern Hemisphere, and this indicates that CO from fossil fuel combustion, which has a large 18O/16O ratio, affects midlatitude in the Northern Hemisphere considerably. On the contrary, the 13C/12C ratio shows a clear seasonal variation with little scatter; maximum −24.5‰ (Vienna Peedee belemnite) in April and minimum −28.5‰ in July-August. The seasonal variation at Happo has different values and phases compared to those in the Southern Hemisphere and in the northern high latitude. 14CO concentration at Happo is similar to that at high latitude in the Northern Hemisphere. A simple box model calculation is presented for these seasonal variations, and the model reproduces the observed seasonal cycles of CO concentration, 18O/16O ratio, and 14CO concentration within the limitations of the simplified model, but not 13C/12C ratio. To reproduce the spring maximum of CO concentration, an enhanced CO production from biomass burning or other CO sources in spring are inferred in east Asia.