Abstract: Transgenic guayule lines were created by constitutively expressing transgenes that encode for prenyltransferase and allylic diphosphate synthase. These new lines are important to rubber production because they result in plants that produce latex rubber which is lower in guayulin, a compound that produces a skin irritation in some animals. Controlling prenyltransferase production also permits the control of the production of resin by the plants, which has important industrial implications. Additionally, prenyltransferase manipulation can result in latex particles of different size, containing rubber of different molecular weight, which is also significant to industrial production. Although the elevated prenyltransferase activity in the transgenic lines resulted in shorter rubber molecules, the number of rubber molecules made by these transgenic lines was increased.