Fumigation of 2 year-old Aleppo pine (Pinus halepensis Mill.) trees with episodes of O3 (up to 120 nl l– as compared to 25 nl l–) throughout two summer seasons produces taller plants with greater stem diameters but reduced root:shoot ratios. Both light and electron microscopy of current year needles carried out at mid-summer reveal extensive accumulations of starch, particularly in the endodermis, and crushing of the phloem sieve cells. Parallel measurements of starch levels also reveal a non-mobilizable component. All these features probably indicate that episodes of high summer O3 detrimentally affect the normal ability of Aleppo pines to withstand severe water stress and this may account for their reduced vitality in Mediterranean regions. By autumn, however, the amounts of starch are similar to those in low O3-grown trees and no evidence of the phloem sieve tube crushing remain although plastids do show increased num bers of plastoglobull. Similar summer high level O3-fumigated needles also show enhanced winter hardiness in the autumn which is unusual for conifers.