A comparative study of leaf and leaflet abscission inAcer negundoandFraxinus americanawas undertaken with special emphasis on leaflet abscission. Leaf fall in both species is accomplished by orderly, fragmentary abscission of leaflets followed by petiole abscission. Leaflet fall was presaged by differentiation of a separation layer at leaflet bases 10–15 days prior to leaflet fall, without an accompanying protective layer. Anatomical studies of petiole abscission revealed early differentiation of a protective layer followed by differentiation of a separation layer at petiole bases just prior to petiolar fall. Abscission at both sites was facilitated by cell division and dissolution of cell walls within separation layers.