KINESIS, a two-headed motor enzyme molecule, hydrolyses ATP to direct organelle transport along microtubules. As it moves along a microtubule, kinesin remains associated with, or (tracks’, microtubule protofilaments(1,2). We have prepared truncated kinesin derivatives that contain either two mechanochemical head domains(3) or only a single head. Unlike intact kinesin and the two-headed derivatives, the one-headed enzyme frequently fails to track protofilaments, suggesting that it detaches from microtubules during movement. In this way, the one-headed kinesin derivative is similar to the motor enzyme myosin, which frequently detaches from the actin filament during movement(4). For myosin (which has two heads), the consequence of this detachment is that single molecules do not appear to drive continuous movement along the filament(5). Our observations suggest that the ability of single two-headed kinesin molecules to drive continuous movement(6,7) results from a ’hand-over-hand’ mechanism(6-8) in which one head remains bound to the microtubule while the other detaches and moves forwards.