Apparatus for interlocking a pair of panels which may constitute the end portions of a wrap around carton blank, one of the panels having an edge strip with at least one locking aperture therein and which is foldably joined to an edge of the associated panel, the other panel being formed integrally with the body portion of a locking tab and an anchoring tab being foldably joined to the body portion of the locking tab, the apparatus including an elongated locking beam having a longitudinal locking groove formed in the lower surface thereof, means for moving a carton and its end panels along the beam and with the panels adjacent the groove, a rotatable strip folder arranged to engage and fold the strip into the locking groove so as to move its aperture out of the plane of the associated panel, a rotatable tab folder arranged to engage and fold the anchoring tab out of the plane of the body portion of the locking element, an abutment disposed in the locking groove downstream from the strip folder and tab folder for engaging and folding the anchoring tab into an acute angle relative to the body portion of the locking element, a pair of guides disposed on opposite sides of the beam for moving the panels and adjacent portions of the carton toward each other thereby to cause the locking element to move into the locking aperture and to cause the anchoring tab to engage the locking edge of the locking aperture.