Melt the butter in a heavy saucepan over low heat; add flour, stirring until smooth. Cook 1 minute, stirring constantly. Gradually add milk; cook over medium heat, stirring constantly, until the mixture is thickened and bubbly. Add rice and the next 6 ingredients; stir well. Remove from heat, and let cool completely.
Beat the egg yolks at high speed of an electric mixer until thick and lemon colored. Add the egg yolks to the rice mixture; stir well.
Beat the egg whites (at room temperature) at high speed until stif peaks form. Fold one-third of the beaten egg whites into rice mixture; gently fold in the remaining egg whites.
Spoon the rice mixture into an ungreased 1 1/2-quart casserole. Bake, uncovered, at 325° for 45 minutes to 1 hour or until a knife inserted in the center of the souffle comes out clean. Serve immediately.