Abstract: A theft prevention device is provided for an anhydrous ammonia tank having a feed pipe with an open end. The theft prevention device includes a plug body which is inserted into the open end of the feed pipe. The plug body includes a flexible member which can be expanded by a keyed lock actuator so as to engage the interior of the feed pipe and prevent removal of the plug member. Locking and unlocking of the plug member to the interior of the feed pipe can be accomplished only by use of a key having a predetermined configuration which fits the keyed lock actuator.