Curtain for a roll up door, particularly for a high speed sliding roll up door of the "self-repairing" type, intended to close or open an opening in a wall delimited by a pair of lateral uprights provided with hollow guide rails for guiding the curtain, as well as by an upper cross member and by a lower plane, both the lateral edges of the curtain being provided with a first elongated flexible member and having a series of substantially rigid sliding blocks fixed to the curtain according to a configuration similar to the teeth of a zipper, which blocks have a cross section substantially corresponding to the cross section of the cavity of said guide rails and incorporate said first elongated flexible member, also a second elongated flexible member substantially parallel to the first elongated flexible member and spaced therefrom towards the outside of the curtain being incorporated in said blocks, characterized in that the cross section of the first elongated flexible member has an area greater than that of the second elongated flexible member, the center of the cross section of the first elongated flexible member being arranged in the half of the body of the relevant sliding block which is closer to the edge of the curtain, the center of the cross section of the second elongated flexible member being arranged in an intermediate position with respect to the half of the body of the relevant sliding block which is the farthest from the edge of the curtain, the distance between the first and the second elongated flexible members (18, 26) being smaller than the diameter of the second elongated flexible member. a high speed sliding roll up door of the "self-repairing" type, intended to close or open an opening in a wall delimited by a pair of lateral uprights provided with hollow guide rails for guiding the curtain, as well as by an upper cross member and by a lower plane, both the lateral edges of the curtain being provided with a first elongated flexible member and having a series of substantially rigid sliding blocks fixed to the curtain according to a configuration similar to the teeth of a zipper, which blocks have a cross section substantially corresponding to the cross section of the cavity of said guide rails and incorporate said first elongated flexible member, also a second elongated flexible member substantially parallel to the first elongated flexible member and spaced therefrom towards the outside of the curtain being incorporated in said blocks, characterized in that the cross section of the first elongated flexible member has an area greater than that of the second elongated flexible member, the center of the cross section of the first elongated flexible member being arranged in the half of the body of the relevant sliding block which is closer to the edge of the curtain, the center of the cross section of the second elongated flexible member being arranged in an intermediate position with respect to the half of the body of the relevant sliding block which is the farthest from the edge of the curtain, the distance between the first and the second elongated flexible members (18, 26) being smaller than the diameter of the second elongated flexible member.