Aircraft observations of ozone and its precursors (NO, NOy, CO, and nonmethane hydrocarbons) were made near Japan (26¿--44¿N) between 21 and 24 April 1998 to investigate the effect of transport and chemistry on the tropospheric ozone over east Asia in spring. The average mixing ratios of ozone and its precursors in the upper troposphere were higher than those observed during February--March 1994. Significantly higher values of CO indicate that the influence of surface sources mediated by convection was large throughout the troposphere during the period. Highly polluted air masses were observed in the upper troposphere at 8--11 km over the Japan Sea on 24 April. These air masses were influenced by cumulus convection associated with a cold front over northeast China about 1 day prior to the observation. However, the majority of observed air masses in the upper troposphere were not directly affected by the recent emissions of the ozone precursors from east Asia. Instead, convection over other regions in the northern midlatitude, followed by long-range transport, affected the abundances of ozone precursors in the upper troposphere. In these air masses, ozone was positively correlated with NOx and C3H8, indicating the effect of photochemical ozone production. The diurnal-average column-integrated rate of ozone production, estimated by a photochemical box model, was larger than the NH average and local stratospheric flux at NH midlatitudes in spring by a factor of 3--20. The net production rates in the majority of the air masses were estimated to be 0.5--4.4 ppbv d-1 in the upper troposphere. These results indicate the important role photochemistry plays in controlling the upper tropospheric ozone abundance in spring.