The transport and chemical transformation of boundary layer ozone in east Asia in January 1997 was investigated using the Models-3 Community Multiscale Air Quality (CMAQ) modeling system with meteorological fields calculated by the Regional Atmospheric Modeling System (RAMS). The predicted ozone concentrations were compared with ground-level observations at four remote sites in Japan and it was found that the model reproduces many of the important features in the observations. Examination of several high ozone episodes indicates that elevated ozone levels are found in association with continental outflow. Net ozone production is found to occur during the daytime over the lower marine atmosphere, and this is also evident from the observations, demonstrating the critical role of the rapid transport of ozone precursors from the Asian continent. An analysis of the ozone budget indicates that the supply and loss of boundary layer ozone in east Asia in winter are dominated by photochemistry.