Abstract: A building block having a six-sided body formed of rigid, foamed, resinous, fire retardant material is bonded on two opposite sides to concrete slabs. Parallel, spaced apart grooves are provided between the body and the slabs and extend along one side of the body and parallel, spaced apart projections are provided between the body and the slabs and extend along another side of the body. The grooves and the projections of each block are so located relatively to the grooves and projections of another block as to interfit with the latter, thereby enabling a wall of any desired length and height to be constructed by interfitting the blocks of each row and course with adjacent blocks. Each of the blocks has one or more openings therein through which a stabilizing and reinforcing member may extend, thereby enabling a wall to be constructed of such blocks without necessitating the use of mortar.