Continuous measurement of CO has been carried out at remote sites in Japan, Oki (36°N), Happo (36°N) and Benoki (27°N). Seasonal variation at each site exhibited a clear cycle with a maximum in spring and a minimum in summer. It has been revealed that the concentrations of CO at Oki are substantially higher than at Mace Head throughout a year. Comparison of trajectory-categorized data between Oki and Mace Head has been made to elucidate that the concentration at Oki is higher than Mace Head both in the clean "background" and in the regionally polluted air mass. The spring maximum at Oki occurs in April that is one month later than the reported data at other remote sites in the northern hemisphere. © 1999 Elsevier Science Ltd. All rights reserved.