Abstract: This invention provides a block to be used with the block means of similar character to create a form for a concrete wall. The blocks are of lightweight plastic and are of substantially rectangular shape, and have opposite ends, opposite sides, upper and lower interlocking surfaces, a substantially hollow interior compartment, and opposite end walls and opposite side walls. The upper interlocking surfaces comprise an elongated longitudinal rib having a rectangular base and an inverted V-shaped upper surface. These ribs have a width less than that of the side walls, and have a substantially horizontal ledge adjacent either side thereof. Lateral extending ribs spaced from each other extend across the ledges and ribs. The lateral extending ribs also have an inverted V-shaped upper surface which extends from a lower rectangular base portion. A plurlaity of cylindrical plugs extend upwardly in spaced relation from the ribs. The lower interlocking surface is a mere image of the upper interlocking surface.