Abstract: A cover for an open enclosure, such as the open rear cargo bed of a truck. The cover is expandable so as to permit the enclosing of materials which extend above the upper edges of the cargo bed sidewalls. This is accomplished by providing a one piece cover which is foldable along two edges to form left and right main panels as well as an intermediate portion which extends below and between the left and right main panels. In an exemplary embodiment, the main panels are held in a closely spaced apart relationship by means of resiliently expandable straps attached to the left and right main panels. When the cover is placed over material which extends above the cargo bed sidewalls, the left and right main panels are pulled apart exposing greater portions of the surplus material forming the intermediate portion of the cover.