Abstract: A lightweight structural member made of prestressed plastic foam is provided, the prestress provided by a plurality of tendons disposed within the foam to which tensile forces are applied during mold casting of the structural member, the tendons being retained in tension by bonding between the tendons and the foam, or by way of anchors which are spaced along the tendons to retain the tendons in immobile condition, one or more skin layers also being optionally provided. A method for producing a lightweight structural member of prestressed plastic foam is also provided.