A plant injury mathematical model, applied previously to acute and chronic leaf injury data, is used here to model National Crop Loss Assessment Network (NCLAN) data for 15 cultivars and to calculate species parameters from the cultivar analyses. Percent crop yield reduction is estimated as a function of a new parameter, the effective mean O3 concentration, where C(sub h) is the hourly average ambient O3 concentration for each daytime hour (defined here as 9:00 A.M.-4:00 P.M., always standard time) of data available at an air sampling site for summer (defined here as June 1-August 31), n is the total number of such available hours, and v is an exposure time-concentration parameter, calculated here to be approximately -0.376. Crop yield reductions for seven plant species are estimated for each of the 1824 siteyears of 1981-1983 hourly O3 concentration data available in the National Aerometric Data Bank (NADB). State-average O3 parameters and estimated percent crop yield reductions are tabulated. Ambient O3 concentrations reduced the total U.S. crop yield an estimated 5% for years 1981-1983.