An anti-theft device prevents tampering with a diesel fuel dispenser. The device bars access to a pulser even if a cover of the dispenser is pried back or removed. A housing formed of plate steel substantially surrounds the pulser and is secured to the frame of the dispenser in three places so that about one-half hour is required to remove the device, even after a cover has been fully removed from the dispenser. This allows ample time for a station operator to observe that a theft is in progress. Three bolts are used to secure the device to the frame of the dispenser. Advantageously, all three of the bolts are placed into pre-existing openings so that no new openings need to be drilled as a part of the installation process. Only one pre-existing bolt of the frame is removed and reinstalled during the installation procedure.