Abstract: A tonneau cover assembly is provided having a rail system, a cover and one or more clamping mechanisms. The clamping mechanism includes a two-piece clamp. The first member is engageable with the rail, and includes an abutment mechanism thereon to contact the rail. The first member also includes a pivotable lever which can be used to exert a force on the second member. The first member includes an opening therein to receive the second member therethrough. The second member includes an extension having one or more teeth thereon. The extension is positioned through the opening in the first member, such that the teeth may engage a portion of the first member. The second member also includes a portion which is engageable with a vertical rail of the side wall of the vehicle. By actuating the lever, the clamp is positioned securely to the rail and the vehicle side wall.