Abstract: The disclosure is directed to filament metering apparatus for use with a rotary lawn mower that uses flexible filament as the cutting element. The filament is wound on a spool that is carried by the engine or motor drive shaft in an axially stationary position. The spool is capable of rotation relative to the drive shaft. A drive member is mounted for rotation with the drive shaft, and is axially movable thereon relative to a driven member carried by the spool. The drive member drivingly engages the driven member in each of two axial positions, and actuating means are included for axially shifting the drive member. The drive and driven members are so constructed that, as the drive member moves from one axial position to the other, relative rotational movement occurs therebetween, which movement is utilized to unwind filament from the spool in a discrete amount.