The invention relates to a header tank for a heat exchanger, comprising a header plate intended for the fixing of a plurality of tubes through which at least one fluid flows, and a cover intended to close the header tank at least in part when fixed to the header plate, and an elastic sealing means (1) situated between the header plate and the cover, characterized in that the sealing means (1) is arranged resting against and in tension on at least two ends of tube. The invention also relates to the heat exchanger that incorporates two header tanks like those defined and to the method of fitting such a sealing gasket in a header tank.
