Abstract: A transmission, in which power is transmitted from one variable diameter pulley to another through an endless band, has an automatic band tensioning mechanism. The band tensioning mechanism includes a tensioning shaft, an adjusting wheel rotatably mounted on the shaft, and a spring connected between the shaft and the wheel. A worm engaged with the wheel is mounted on a shaft slidably received in a locking member. The worm has a locking shoulder at one end which is normally urged, by the spring acting through the wheel, into engagement with a locking shoulder on one end of the locking member. The worm is moved out of locking engagement against the bias of the spring for rotation of the worm and wheel, and tightening of the spring.