The physiological drop of immature fruits was studied in relation to the activation of the abscission zone located between the fruit and the receptacle. Light- and electron-microscopy observations demonstrated that this zone consisted of two types of parenchymatous cells: in the distal region, closer to the fruit, were groups of small thick-walled cells with few intercellular spaces; in the proximal region, closer to the pedicel, the stillgrouped cells were larger, polyphenolic-rich, and thick-walled but with many wide intercellular spaces. Separation of the fruit occurred by dissolution of the middle lamella of the cells of this zone followed by an increase in the size of the intercellular spaces. Lysis of the middle lamella began at the corners of the cells and spread from there across the entire wall surface. Structural changes were paralleled by an increase in soluble proteins, endo-cellulase and exo-polygalacturonase activity. Isoelectric focusing indicated that both enzymes were present as isoenzymes whose patterns were affected by embryoctomy and 2-chloroethylphosphonic acid treatments.