Ultrafine single-crystalline gold nanowires (see figure) are synthesized by oriented attachment of nanoparticles in the solution phase. An amine capping agent is preferentially removed from the {111} planes, which causes the wires to form by fusion along these planes. Such nanowires can be used as the fundamental building blocks for nanoelectronics, and will enable basic studies on electric transport in one-dimensional structures to be carried out.