The natural drop of olive fruit, before harvest, in relation to stages of fruit development, was studied in Manzanillo olives. The absolute amount of green and half-black fruit drop was rather constant for most of the studied period. The drop of black fruit increased constantly with the advancement of maturation on both an absolute and relative basis. In studying the effect of various halogenated acetic acids on the abscission induction of Manzanillo and Souri olives, no consecutive order of activity could be found. Dichloroacetate (DCA) and monoiodoacetate (MIA) showed the highest activity. A pretreatment of Manzanillo olives with 2,4,5-chlorophenoxypropionic acid (2,4,5-TP) and chlormequat (CCC) did not affect the efficiency of various abscission-inducing chemicals. Large-scale mechanical harvesting trials with the Souri variety showed a mean increase of fruit drop up to 26% when agents such as glycerin, MIA and 2-chloroethane phosphonic acid (CEPA) were used over a period of three seasons. © 1973.