A double seat valve for aseptic lines for product is described, the valve comprising a valve housing, two line ports arranged on the valve housing for connecting the aseptic lines for product to the valve housing, two valve seats that are formed between the line ports on the valve housing and confine a safety chamber formed therebetween for leaks against the line ports, a first closure element which is actuatable by way of a first valve stem and acts against one of the two valve seats, and with a second closure element which is actuatable by way of a second valve stem and acts against the other of the two valve seats. T first valve stem at least in part surrounds the second valve stem, so that both are movable independently along a valve axis. The first valve stem is connected to the second valve stem by way of a diaphragm seal formed in a ring-shaped manner around the valve axis A for sealing against the safety chamber, characterized in that an outer fastening portion of the annular diaphragm seal is bent inwardly toward the valve axis by a bending angle of more than 90°, so that it abuts against on a tapered surface of a first fastening element.