Abstract: The closure system according to the invention is intended in particular for plastics immersion pumps which in turn have a pump casing which has a substantially cylindrical casing wall, a detachable casing cover and a locking ring which is intended for closing the casing cover and, in the mounted state, is arranged on the outside of the casing cover and is inserted in an annular groove of the casing wall. According to the invention, the casing cover is provided on its outside with at least two ribs which are arranged in a spoke-like manner and each have an outer projection and with this each expose a groove radially accessible from the outside, and the locking ring is provided with the same number of radially inward projecting tabs. The latter are dimensioned and arranged in such a way that, in the assembled, i.e. operational, state of the pump, they are each inserted in a tightly fitting manner into a groove of the ribs arranged at the end face of the casing cover.