Abstract: A swimming pool cover having at least a portion of which is made of material allowing solar energy to pass therethrough coacts with a solar heating mat in the pool to retain heat developed by the mat. The dark-colored mat has a specific gravity sufficiently greater than that of water to allow the mat to sink to the bottom of the swimming pool. The mat is made of a pliable material and has a crossbar member to facilitate rolling of the mat so that the surface area of the mat may be varied in order to provide control of the temperature of the water. A cylindrical air sac is connected to the cover, which is made of pliable material, and is positioned adjacent an edge of the cover in order to allow rolling and unrolling of the cover and is sufficiently buoyant to support the cover in the water when the cover is in rolled and unrolled positions and during movement therebetween.