Julia Roberts is making another debut: this time as a spokeswoman for alternative fuel.

Dallas-based Earth Biofuels announced that Roberts will become a spokeswoman for the company and chair its new advisory board.

"It's very important that we expand our use of clean energy and make a long-term commitment to it. Biodiesel and ethanol are better for the environment and for the air we breathe," Roberts said in a statement released this week.

Roberts joins fellow Oscar winner Morgan Freeman and country singer Willie Nelson in promoting the use of renewable fuels. Freeman and Nelson are on Earth Biofuels' board of directors.

Earth Biofuels produces biodiesel and is the exclusive distributor of Nelson's signature brand of biodiesel. The soy derivative is mostly sold in a blend called B20, which is 20 percent biodiesel and 80 percent diesel.

Earth Biofuels said it will work with Roberts on a program to encourage the use of biodiesel in more than 500,000 diesel school buses nationwide.

The 38-year-old actress made her Broadway debut this spring in a revival of Richard Greenberg's "Three Days of Rain."

Her screen hits include "Pretty Woman," "Steel Magnolias" and "Erin Brockovich" -- for which she won an Oscar in 2001. She gave birth to twins, Hazel and Phinnaeus, in November 2004. She is married to cinematographer Danny Moder.