A riding lawn mower includes a belt-drive mechanism for transferring power from a drive pulley wheel to a deck pulley wheel to rotate cutting blades of a height-adjustable mowing deck. The drive pulley wheel connects to a drive shaft of a mower engine at a rear portion of the mower and rotatably engages a drive belt. The deck pulley wheel connects to the mowing deck at a location longitudinally spaced from the drive pulley wheel, and rotatably engages the drive belt. A guide pulley wheel, having a circumferential groove for retaining the drive belt, is located between the drive pulley wheel and the deck pulley wheel. An elongated arm pivotally connects at one end to the mower deck near the deck pulley wheel and at an opposite end pivotally engages a mounting bracket connected to the mower frame near the drive pulley wheel. The guide pulley wheel rotatably mounts to the arm near the pivotal connection of the arm to the mower deck. The arm registers angular displacement of the drive belt relative to the drive pulley wheel and adjusts the guide pulley wheel to rotate in a plane representative of the angular displacement of the drive belt.
