In a mixing bowl, combine rhubarb, sugar, and flour.
Place one pie crust in a pie dish. Pour the rhubarb mixture into the crust and dot with butter.
Cover with the second pie crust. Seal the edges and cut small slits in the top crust to allow steam to escape.
Bake for 15 minutes. Reduce the oven temperature to 350°F (175°C) and continue baking for 40-45 minutes, until the crust is golden and the filling is bubbly.