Abstract: A mold box and mold head that are interfitted together to mold concrete blocks. The mold box has two sides and two ends forming a continuous flange around it and two laterally spaced partitions equally spaced between the two ends, one partition being in the form of a straight bar, and the other, in the form of three spaced boss members. Upwardly-extending bosses that are generally plate-like are fixed to rods which extend across laterally, parallel to the ends. The head is made up of a plate-like member having spaced plate-like members attached to it. The spaced plate-like members cooperate with the partitions in the mold box to form a concrete block therein. The resulting block is tongued on one side and grooved on the other and the top is tongued and the bottom grooved for interlocking purposes. The interior section contains three rows of cells providing greater insulation and saving of material.