A Filter element for insertion into a housing of a ventilation unit or air conditioner, comprising a bellows with folds and a front side, which can face a wall of the housing, wherein the folds on the front side define fold cross-sectional surfaces, wherein to the front side at least one guiding slot is assigned and wherein a fold cross-sectional surface has a geometry in the area of a guiding slot that is different from the fold cross-sectional surface of a multiplicity of folds, is, in view of the task of providing an efficient filter element which, after problem-free manufacture is equipped with guiding slots into which guide pins can be inserted, characterized in that a multiplicity of regular fold walls is provided, wherein each two flanking walls flank the guiding slot.