Abstract: There is disclosed a mold assembly for casting a concrete wall having an opening and for anchoring a flexible resilient ring-like gasket in the peripheral wall of said opening to make a seal with a pipe passing through the opening. The assembly includes a pair of spaced mold walls for receiving unset concrete therebetween, a flexible resilient annular mold arranged to extend between the mold walls with its end surfaces in respective sealing contact with the inner surfaces of the mold walls and a rigid mold-supporting member removably inserted within the annular mold to support the latter while concrete is cast about the annular mold and between the mold walls. The outer surface of the annular mold has a continuous annular groove, intermediate the end surfaces, to receive the pipe-engaging portion of the gasket. Once the concrete is set, the outer anchoring portion of the gasket is embedded in the concrete.