Abstract: The present proposal comtemplates an anti-theft device for unattended trailers, in the nature of a cover-plate assembly for locking the open end of the trailer tow bar to prevent unauthorized removal of the trailer as by connecting the open end of the trailer bar to a peg on a towing vehicle. The assembly may comprise an underplate with a centrally disposed pintle and an apertured overplate or coverplate constructed and arranged to coact with the underplate to effectively prevent access to the opening at the end of the tow bar. For convenience, the two plates may be joined in spaced relationship as by a steel cable fixed at its ends, respectively, to the top side of the coverplate and to the reverse side of the underplate.