Abstract: An improved insulating block form for use in the construction of concrete wall structures wherein the block form is formed from expandable polystyrene material to provide a lightweight, rigid, box-like structure having a pair of oppositely disposed side walls and end walls which together define a body cavity to receive concrete therein. A plurality of transverse strut members integrally support the side walls and further define a plurality of cells. Each end wall includes a pair of inserts which are adapted to be mounted to the transverse strut member when the elongated block form is required to be cut for a particular installation. The side and end walls are further provided with interlocking members whereby they are readily stacked one above the other and side-to-side in a secure interlocked arrangement.