Cook ziti according to directions; drain. In a saucepan, brown sausage. Remove to paper towels to drain.
Melt butter in skillet and saute mushroom, green pepper and onion until just tender. Blend flour into the vegetable mixture, stirring until well blended and bubbly.
Gradually add milk and cook over medium low heat, stirring constantly, until mixture begins to boil. Continue to cook and stir for 1 minute. Blend in 1 1/2 cups cheddar cheese. Parmesan cheese and pepper, stirring until cheeses are melted and mixture is smooth.
Stir hot, cooked ziti and sausage into vegetable cheese sauce. Pour into greased 2 or 3 quart casserole. Top with remaining 1/2 cup cheddar cheese. Bake at 350 degrees for 30 minutes or until hot and bubbly.