In exposure-response modeling, a major concern is the numerical definition of exposure in relating crop loss to O3, yet few indices have been considered. The paper addresses research in which plant growth was regressed for soybean, wheat, cotton, corn, and sorghum against 613 numerical exposure indices using the Box-Tidwell model. The top-performing generalized, phenologically weighted, cumulative impact indices accumulated concentrations emphasizing O3 concentrations of 0.06 ppm (118 micrograms/cu m) or higher, and had phenological weighting schemes with greatest weight occurring 20 to 40 days prior to crop maturity. These findings indicate that (1) peak concentrations are important, but lower concentrations should be included in the calculations, (2) increased plant sensitivity occurs between flowering and maturity, and (3) plants respond to cumulative exposure impact. (Copyright (c) Elsevier Science Publishers B.V.)