Burger King says it's rolling out its vegan Impossible Whopper to more menus and aiming for a national rollout by the end of 2019.

The burger chain kicked off its Impossible Burger test on April Fools' Day, serving the vegan version of its iconic burger at 59 locations in and around St. Louis, Missouri.

"The Impossible Whopper test in St. Louis went exceedingly well and as a result there are plans to extend testing into additional markets in the very near future," a Burger King representative said in a statement on Monday.

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Burger King's vegan burger is taking over America.

On Monday, the burger chain said the plant-based Impossible Whopper would roll out to more regions this summer and, if customer enthusiasm continues, the entire country by the end of the year.

"The Impossible Whopper test in St. Louis went exceedingly well and as a result there are plans to extend testing into additional markets in the very near future," a Burger King representative said in a statement.

Burger King kicked off its Impossible Burger test on April Fools' Day, serving the vegan version of its iconic burger at 59 locations in and around St. Louis, Missouri. The burger chain worked with Impossible Foods, a company that creates plant-based meat substitutes — including its signature Impossible Burger — to create the Impossible Whopper.

Read more: Burger King is testing a plant-based Impossible Whopper

On Monday, the burger chain said that the St. Louis test was showing "encouraging results" and that sales of the Impossible Whopper were "complementing traditional Whopper purchases."

José Cil, the CEO of Burger King's parent company, Restaurant Brands International, said on a call with investors that the reaction so far had exceeded the company's expectations.

"We aren't seeing guests swap the original Whopper for the Impossible Whopper," he said, adding that "it's attracting new guests." Cil said it was difficult to taste a difference between the two burgers.

Fast-food chains are increasingly working to win over vegan and vegetarian customers. Taco Bell is testing a 100% vegetarian menu, and Chipotle recently added vegetarian and vegan items to its menu.