The invention relates to a fuel cell stack (10) of a fuel cell system that is to be hermetically sealed off from the environment after operation is ended so that only a finite amount of air oxygen remains in the fuel cells, thus preventing the fuel cells from excessively aging. According to the invention, locking means (26, 28; 32, 34) are provided that are disposed on or in an end plate (14) or distributor plate of the fuel cell stack (10). The locking means (26, 28; 32, 34) are therefore thermally coupled to the end plate (14) and may freeze up at cold ambient temperatures. In a preferred embodiment, the locking means (26, 28; 32, 34) comprise an elastomer body (32, 34) able to cover and uncover a channel (44, 46) in the plate (14).