Expts. are reported which suggest that defoliation of the tomato plant is in part a response to emanations of ethylene from ripe fruits underneath the vines. Young potted tomato plants placed in glass-inclosed chambers with a number of ripe tomato fruits showed epinasty followed by leaf abscission, which is a typical response to ethylene. Young plants transplanted into a tomato field in full bearing and near ripe fruits defoliated; similar young plants transplanted a short distance from the field remained normal. Older undefoliated plants, including a so-called "bull" plant, i.e., a non-bearing and normally non-defoliating plant, were induced to defoliate by placing ripe fruits underneath their vines.