Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13 inch baking dish.
Cook macaroni according to package directions; drain.
In a medium saucepan, melt butter over low heat. Stir in flour and dry mustard. Gradually stir in milk until smooth. Cook over medium heat, stirring constantly, until mixture thickens and boils. Boil and stir for 1 minute.
Remove from heat. Stir in cheeses until melted.
In the prepared baking dish, layer half of the macaroni and half of the sauce. Repeat layers.
Bake in preheated oven for 30 minutes, or until bubbly and lightly browned.