A hearing aid has a housing, a battery compartment, and a locking mechanism for the battery compartment. A shaft which is arranged on the housing or the battery compartment, and a slide having a hole which forms a fit with the shaft, such that the slide can be moved on the shaft in order to lock and unlock the battery compartment in the housing. The hole has a circular cross section, and the shaft has a cross section which is not a circle, with rotational symmetry when rotated through 360°/n, where n is an integer greater than 1. A battery compartment for a hearing aid has a locking mechanism such as this. Because a cross section of said shaft is different from the cross section of the hole in the battery compartment, the shaft is connected to the hole in the battery compartment only at a number of points. This ensures that the slide is held adequately on the shaft even in the event of a production tolerance on the one hand, while, on the other hand, the slide can move on the shaft with little resistance. In a preferred embodiment the shaft is trilobular in cross-section.