Abstract: A cover assembly for open boxes of vehicles is mounted in the open box by being secured at least at corners of a first end thereof at a first end of the open box. A pair of substantially rigid guides is mounted, one on each side wall of the box near the top edge thereof, and the cover is mounted between the guides with a plurality of hooks or other suitable means secured along each side of the flexible cover material so as to be slidable on the guide means. Fasteners, such as rubber shock cords, are provided to secure the second end of the cover remote from the first end at the second end of the open box remote from the first end when the cover is extended along the length of the box. Indicator means such as flags are provided at the second corners of the cover to indicate whether the cover is extended along the box or not. The guide means are preferably a pair of taut cables.