Provided in this disclosure is an antitheft system for an automotive catalytic converter including a wire loop for conducting a current to establish an electrical circuit. One or more hose clamps are provided for securing some or all of the wire loop alongside a vehicle exhaust system including the catalytic converter. A monitoring circuit is in electrical communication with the wire loop, for monitoring the electrical circuit established in the wire loop. A detector circuit is connected to the monitoring circuit, for detecting a discontinuity in the electrical circuit indicative of a disconnection of the wire loop. An alarm circuit is connected to the detector circuit, for activating an alarm upon detection of the discontinuity in the electrical circuit. An arming circuit includes a detection circuit for detecting insertion of a removable key, and another detection circuit for detecting insertion of the wire loop which establishes the electrical circuit.