Abstract: By a method and an apparatus for cleaning water in a swimming pool, the water is circulated successively through a plurality of flow chambers divided by partitions. In the front chamber bar-shaped heating bodies are located transverse to the flow direction of the water, and in the succeeding chamber ultraviolet radiation lamps are located transverse to the flow direction, said lamps radiating light at .lambda.>300 nm. In a last chamber ultraviolet radiation lamps are located which radiate light at .lambda.<300 nm. When the water passes through the container, skin squames and the like organisms collect on the heating bodies, whereas the ultraviolet radiation lamps in the chamber ensure such a disinfection that only a quite small amount of fresh water need to be supplied to the swimming pool. The ultraviolet radiation lamps serve to kill the bacteria resistant to chlorine.