Abstract The growth regulators abscisic acid (ABA), 6-benzylamino purine (BA), (2-chloroethyl)phosphonic acid (ethephon), aminoethoxyvinylglycine (AVG), and silver nitrate were applied in weekly or biweekly foliar sprays to watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] plants grown in a greenhouse. Both 10−3 mBA and 10−4 m ABA inhibited stem elongation, but had no effect on flowering patterns. Ethephon at 60 ppm and above prevented flower development, and at 15 and 30 ppm inhibited pistillate flowering and increased staminate/pistillate (S/P) flower ratios fivefold to sevenfold. Grafting watermelon scions to rootstocks of gynomonoecious and andromonoecious muskmelon did not affect sex expression or the response of watermelon to ethephon. AVG at 100 and 200 ppm and 500 ppm AgNO3 reduced the number of staminate flowers and promoted hermaphroditic flowering. In watermelon, contrary to other cucurbits, applied ethylene appears to suppress rather than promote ovary development during flower bud differentiation.