Abstract: Greenhouse production of seedling transplants of leafy green vegetables usually requires the use of chemical sprays to avoid pest infestations that will be transferred and multiplied in the field. We investigated the use of trinexapac-ethyl (TE), a gibberellin synthesis inhibitor, as a seed treatment to induce resistance to whitefly (Bemisia tabaci, WF) in romaine-type lettuce seedlings grown in growth chambers or screenhouses. Leaves of young (2-3 wk old) lettuce seedlings grown from seeds treated with 1 mM TE had fewer WF eggs or nymphs, while older (4-5 wk old) seedlings had 30% fewer WF pupae and 50% fewer adults. Leaves of seedlings from treated seeds had greater amounts of carotenoids and anthocyanins, more antioxidative capacity, and thicker leaves and cuticular layers than those of controls. These TE-induced changes in leaf physiology or morphology provided resistance to WF attack and could reduce the environmental hazards of pesticide application.