Four weeks after starting a pilot in St. Louis for the plant-based burger, Burger King announced Monday it intends “to quickly test in additional markets with the intention of nationwide distribution 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,” the company said in a statement.  “Burger King restaurants in St. Louis are showing encouraging results and Impossible Whopper sales are complementing traditional Whopper purchases.”

The additional test markets and timeline were not immediately available, but the plant-based Whopper developed by Silicon Valley-based Impossible Foods continues to be available at 59 Burger King restaurants in and around St. Louis.

Burger King is not alone in adding vegan and meatless options.  Del Taco launched its new Beyond Taco and Beyond Avocado Taco on April 25, which includes a special Beyond Meat protein.  Fast-casual pizza chains are also incorporating Beyond Meat protein as well as vegan cheese.