Disclosed is a speed control mechanism (12) for controlling selective engagement of the self-propelled transmission assembly for a walk-behind lawn mower. The speed control mechanism (12) controls the relative output speed of the transmission assembly. The speed control mechanism (12) includes a speed engagement assembly and a speed adjustment assembly. The speed engagement assembly includes a pair of rotatable levers (34,36) coupled to a casing for causing the transmission assembly to actuate between a disengaged position and an engaged position. The speed adjustment assembly includes a rotatable knob (42) for causing the relative output speed of the transmission assembly to the wheels of the lawn mower to increase or decrease.
