The development of the walls of the parenchyma cells of apple fruit cortex has been followed throughout two seasons under conditions which allow an assessment of wall structure as a factor involved in the keeping quality of apples. Changes in chemical compositions, in physical organization, and in cell size have been monitored throughout.Unexpectedly rapid wall synthesis occurs during cell extension but is followed, before harvest, by a period during which some pectic substances are reassimilated and in which some lability of wall components is indicated. There is a marked thinning of the wall at this stage, for which subsequent secondary thickening fails to compensate fully.Differences in cell-wall structure between apples from trees receiving different manurial treatments are most marked during cell extension, when cells from apples which keep well show symptoms consistent with a lower nitrogen level. These differences have largely disappeared by harvest, but reasons are given to indicate that the earlier stages of fruit development may have a considerable influence on potential storage life.