Abstract Significant variation in tolerance to ozone (75 pphm) was expressed among red maple seedling progenies from four seed sources. Differences in seedling response occurred in extent of injury but not in type of injury. Seedlings from an Alabama source showed the least average foliar injury and least suppression of leaf growth after exposure to ozone in four stages of growth and four stages of leaf development. These Alabama seedlings also exhibited the largest increase in height growth after each of the four fumigations. Foliar injury and suppression of leaf growth was greatest in Pennsylvania and Minnesota seedlings. Despite a high degree of foliar damage, the Pennsylvania progeny showed significant retardation of growth in only one plant-growth stage, when seedlings were youngest. In all sources and in all plant-growth stages, the youngest (