KINESIN is a ’motor’ molecule, consisting of two head domains, an alpha-helical coiled coil rod, and a tail part that binds to its cargo(1,2). When expressed in a bacterial system, the head domain is functional(3), and can bind to microtubules with the stoichiometry of one head per tubulin dimer. Kinesin moves along microtubules by means of a cyclic process of nucleotide binding, hydrolysis and product release(4,5). We have used negative-stain electron microscopy and image analysis to study the structures of microtubules and tubulin sheets decorated with the motor domain (bead) of kinesin in three states : in the presence of an unhydrolysable ATP analogue, 5’-adenylylimidodiphosphate (AMP-PNP); without nucleotides; and with adenosine 5’-diphosphate (ADP). A single kinesin head bound to a microtubule has a pear-shaped structure, with the broader end towards the ’plus’ end of the microtubule under all conditions; the reverse motor, ncd, is similarly oriented. Three-dimensional maps reveal that kinesin heads have a spike that is assumed to form the attachment to the tail of a complete kinesin molecule. This spike is perpendicular to the microtubule axis in the presence of ADP, but points towards the plus end (similar to 45 degrees) in the presence of AMP-PNP or absence of nucleotides. Our results provide direct evidence for a conformational change of the kinesin motor domain during the ATPase cycle.