Preheat oven to 350° F.
In a large bowl combine sugar, lemon rind, and lemon juice. Mix well, then stir in melted butter or margarine and egg yolks. Mix in flour. Slowly pour in milk while stirring. Mix until all ingredients are thoroughly combined.
In a medium glass or metal bowl, beat egg whites until soft peaks form. Fold into milk mixture. Pour mixture into pastry shell.
Bake in preheated oven for 25 to 30 minutes, until golden brown on top.