Implementing ozone (O3) standards on regional air sheds on the basis of crop losses requires more precise analysis than statewide or even national assessments. An economic model that measures crop losses by air basins in California is described. The ability of the model to respond to subtle regional changes in O3 standards is tested. Substantial differences in economic benefits between different standards and regions result from the model simulations. Regional standards are shown to capture a large proportion of the benefits at a lower cost.