An improved automatic height adjustment system for use with a header of an agricultural harvester is provided. The system includes apparatus and methods utilizing detection of a light beam at or near the skid plate of a header to adjust the height of the header relative to the ground surface. The apparatus includes an emitter for emitting a beam of light to a receiver. An apertured sensor guide is positioned between the emitter and the receiver. The sensor guide is in communication with a controller. If the body of the sensor guide is detected as blocking passage of light to the receiver then the controller operates an extensible actuator to raise or lower the header relative to the ground surface. The system provides rapid header height adjustment in response to detected changes in elevation of the skid plate as the harvester traverses a crop field.