Spokesman Shawn Howard says the company began filling the US$2.3-billion line with oil Saturday morning in Cushing, Okla.
That crude will eventually make its way to Houston-area refineries.
Howard says over the coming weeks, TransCanada will inject about three million barrels of oil into the system.
TransCanada is still awaiting approval from the Obama administration to construct the US$5.4-billion northern portion of Keystone XL more than five years after it applied for a permit.
The company was able to move ahead with the Gulf Coast portion first because it doesn't cross the Canada-U.S. border.
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