Abstract: An interlocking building block system comprises a plurality of rectangular building blocks, each with longitudinal side surfaces, upper and lower longitudinal faces, and vertical end walls normal to the side surfaces and longitudinal faces. The upper longitudinal face of the blocks has a central longitudinal arcuate rib and a pair of transverse spaced arcuate ribs thereon. The transverse ribs extend laterally from the longitudinal rib to one of the side surfaces. Corresponding longitudinal and lateral transverse concave arcuate recesses in the lower longitudinal face mate with the ribs of an adjacent block when assembled. A pair of spaced vertical slots extend through the block inwardly of each end wall opposite to the transverse ribs and terminate tangentially to the longitudinal rib.