KINESIN is a microtubule (MT)-associated ’motor’ molecule fundamental to organelle transport(1,2). Recently, various kinesin superfamily members (KIFs) have also been identified and suggested as being responsible for the transport of specific organelles(3-5). Kinesin is a heterotetramer composed of two heavy chains and two light chains. The heavy chains form two globular heads, a rod and a fan-like tail completed by the light chains(6,7). The globular head, which is composed of approximately 340 amino-terminal residues of the heavy chain, includes both ATP-binding and MT-binding domains, and its recombinant protein also has these properties(8). To improve the understanding of the mechanism of force generation by an MT-based molecular motor, kinesin, we report here the three-dimensional structure of the complex of a recombinant kinesin head and MTs, as revealed by helical reconstruction from cryoelectron micrographs. A kinesin head is a globular teardrop-like structure binding to the ridge of one protofilament of MTs. We have determined the polarity of the structure of the complex of MTs and the kinesin head in relation to MT polarity.