An adjustable gathering chain assembly includes a drive unit, a driven unit spaced from and movable with respect to the drive unit and an endless gathering chain extending between the drive unit and the driven unit. An adjustment unit including an idler member in engagement with the gathering chain is operable to move the idler member with respect to the drive unit in order to move the driven unit between first and second positions relative to the drive unit whereby the header may effectively harvest both standing crop and downed or lodged crop depending on harvesting conditions.