Preheat oven to 350 degrees F. Grease and flour two 9 x 5 x 3 inch loaf pans.
Set aside cocoa packet from mix. Combine cake mix, pudding mix, eggs, water and oil in large bowl. Beat at medium speed with electric mixer for 2 minutes. Measure 1 cup batter. Place in small bowl; stir in contents of reserved cocoa packet.
Spoon half the yellow batter in each loaf pan. Spoon half the chocolate batter on top of yellow batter in each pan. Run knife through batters to marble.
Bake at 350 degrees F for 45 to 50 minutes or until toothpick inserted in center comes out clean. Cool in pans 5 minutes. Carefully loosen cakes from pans. Invert onto cooling racks. Cool completely. Cut cakes into slices.