Abstract: An automatic power transmission belt tensioner having spring structure for providing a dual biasing of an idler roller against the power transmission belt. The biasing structure provides a dual biasing of the arm carrying the idler roller including a biasing under torsion and a biasing under compression of the spring structure. In one form, a pair of biasing springs is utilized, one for providing the torsion biasing and one for providing the compressional biasing. In another form, a single spring effects both of the dual biasing actions. The compressional biasing structure includes a pair of cams having cooperating inclined surfaces for effecting compression of the compression spring as a function of the movement of the idler roller arm.