Abstract: A pet litter box, including a device for locking the box after it has been used and re-closed, is formed from a single foldable blank. When the box is closed or re-closed it has inter-laying first and second halves, each having bottom and sidewall portions. The halves are connected to one another by a joining portion, and joining sidewalls. Each of the sidewalls of the first half carries a tab. The sidewall portions of the second half each define a diagonal crease, and the pair of joining side-wall portions each define a diagonal crease and a pair of vertical creases. When the box is fully open its first, second and joining portions define an open tray. After the system has been used the box can be re-closed and locked. During re-closing the joining sidewall portions and the two sidewalls of the second half all fold inwardly at their creases, thereby forming a pair of slots between the pairs of creases on each side.