Abstract: A kit for assembling a toolbox adapted for installation in the back of a pickup truck, the kit containing: a preassembled upper section, an unassembled lower section, and fasteners for fastening the upper section to the lower section, when assembled. The interrelatable parts of the unassembled lower section include: a bottom panel, a pair of end panels, a pair of side panels, and four locking members. The end and side panels carry tabs for interconnecting with corresponding slots on the bottom panel. Adjacent side edges of the end and side panels have abutting locking channels which are slidingly interlocked by the locking members.