To investigate the role of ethylene in sex expression by cucumber (Cucumis sativus L.) in vitro, whole seeds of �Burpless Hybrid� were explanted onto modified Murashige and Skoog (MS,1962) medium supplemented with various concentrations of Ethephon (2-Chloroethyl phosphonic acid, Ethrel�), an ethylene releasing compound (0.0, 1.0, 2.0, 5.0, and 10.0 mg/l). Exogenous Ethephon significantly increased the incidence of female flowers and reduced the number of male flowers, especially at 10.0 mg/l (8.5 female flowers/tube vs.0.0 female flowers/tube for control). Control explants (0.0 mg/l Ethephon) produced only male flowers (2.8 male flowers/tube). This information will be useful for researchers interested in studying flower induction, development, and sex regulation in vitro.