Abstract: Adjustable ventilation apparatus for a lawn mower, adapted to allow adjustment of the airflow rate into the cutting deck to maximize operation of the lawn mower under various operating conditions. In one embodiment, the apparatus comprises a ring disposed intermediate the engine and cutting deck of a walk behind mower, having a plurality of radial openings formed therethrough. An annular cover is provided encircling the ring, with the cover having a like number of openings formed in the peripheral wall thereof. Rotational movement of the cover relative to the ring moves the openings therein into and out of alignment with the openings in the ring, thereby defining open and closed positions for the vent, respectively. The cover may also be moved to a plurality of positions intermediate fully open and fully closed to provide the appropriate airflow into the cutting deck.