THE NEW GROWTH REGULATOR, Ethrel (2-chloroethyl phosphonic acid), is similar in action to ethylene in its effects on various plant processes: it accelerates post-harvest ripening of tomato, banana, and honeydew melon fruits; it induces flowering in pineapple plants; it causes female flower differentiation in cucumber plants; and it acts as a thinning agent by accelerating abscission of flowers and young fruit in certain trees and by loosening fruit at harvest to aid mechanical harvesting. These experiments were initiated to examine the effects of Ethrel on growth and maturation of tomato fruit on the vine under both greenhouse and field conditions.