Heat oven to 350°F. Grease and lightly flour 1 1/2-quart soufflé dish or casserole. Make 4-inch band of double thickness foil 2 inches longer than top edge of dish; grease one side. Place foil, greased side in, around top edge of dish extending above edge. Secure foil with clear tape.
Melt margarine in large saucepan over medium heat. Add flour, dry mustard and salt; cook and stir until bubbly. Gradually add milk; cook until mixture is very thick and smooth, stirring constantly. Add cheese and Worcestershire sauce; stir until cheese melts. Remove from heat. Add egg yolks 1 at a time, beating well after each addition.
In medium bowl, beat egg whites just until stiff peaks form. Fold into cheese mixture. Carefully pour into greased and floured soufflé dish.
Bake at 350°F. for 55 to 60 minutes or until knife inserted near center comes out clean. Carefully remove foil band. Serve immediately.