Preheat oven to 400° F.
Mix together the sugar and flour. Add the lemon juice and rind, egg yolks, butter and salt. Mix well. Mix in the milk. Beat egg whites until firm; fold into batter. Pour into prepared pie crust.
Bake for 10 minutes, then reduce oven temperature to 350° F. and bake for
another 30 minutes or until pie springs back to a light touch. Cool on rack.