Abstract: An automatic tensioner to be used with a drive train of a self-propelled mower for drivingly connecting a ground wheel to the mower transmission, wherein the tensioner includes tensioning mechanism, which may be constructed as a spring biased idler arm having an idler member rotatably mounted thereon, to apply a tensioning force against the flexible element of the drive. The tensioner further includes an anti-slackening assembly having an elongated takeup rod being pivotally coupled with the idler arm at one end thereof so that the takeup rod moves generally with the idler arm, as the arm moves in a tensioning direction and opposed slackening direction. The anti-slackening assembly also includes a swingable control tab having an aperture receiving the other end of the takeup rod.