Abstract: An anti-backoff screw cap construction for a container neck having a thread with a downwardly-facing pressure face and an upwardly-facing relief face. The cap has a depending skirt having an internal thread with an upwardly-facing pressure face for engagement with the downwardly-facing pressure face of the container neck thread. Engagement results in tightening of the cap on the container neck when the cap is turned in a screwing-on direction. The cap thread has a downwardly-facing relief face having as an anti-backoff structure, an elongate, substantially helical, resilient projecting bead which is integral with the cap thread, the bead being coextensive with an uppermost portion of its downwardly-facing relief face and being directly engageable by the upwardly-facing relief face of the container neck thread. The projecting bead is resiliently deformed by its engagement with the upwardly-facing relief face of the container neck thread when the cap is turned in its screwing-on direction.