A one-piece water-closet gasket (21) comprises an outer gasket (39) for the stand (41) and an inner gasket (43) for sealing the outlet of the water-closet drain-duct (29) to the sewage inlet duct (37). The latter gasket comprises a tube (51) having a lower span (53) for plugging into the sewage inlet and is provided with means (55, 57) elastically bearing and sealing against the inner wall surface of the sewage duct.