Locking structure for thermoformed plastic containers for food and other articles wherein the lid and the base of the containers are provided with locking means, specifically a detent and a tab. The detent, which depends from the front of the lid, has rounded ends and an inwardly extending lip disposed on its outer side. The tab, which depends from the front of the base, has an upper surface, a lower surface, and two end surfaces. The upper surface is angled toward the lower surface and the end surfaces are substantially flat. The upper surface and the inwardly extending lip are constructed and arranged with respect to each other so that when the detent is pressed into the tab the inwardly extending lip will snap into position beneath the upper surface and interlock with it to lock the lid and base.