The ATPase mechanism for a monomeric Drosophila kinesin construct, K341, was determined by pre-steady-state kinetic methods and compared to dimeric kinesin, K401. We directly measured the kinetics of binding mantATP (a fluorescent ATP analog) to the microtubule K341 complex, the dissociation of K341 from the microtubule, and release of phosphate and ADP from K341. Measurements of phosphate release kinetics at low salt concentration show that K341 hydrolyzes 18 molecules of ATP per kinesin monomer prior to release from the microtubule. At a higher salt concentration the amplitude of the pre-steady-state burst of phosphate release was reduced to 8 molecules per kinesin monomer. The maximum rate of dissociation of K341 from the microtubule following the addition of ATP was 22 s-1. The rate of mantADP release from the M.K341.mantADP complex increased as a function of tubulin concentration with a second-order rate constant of 11 microM-1 s-1 for K341 binding to the microtubule and reached a maximum rate of mantADP release of 303 s-1. ADP release kinetics were also determined by monitoring the binding of mantATP to K341.ADP and K401.ADP after mixing with microtubules. We show that monomeric kinesin remains associated with the microtubule through multiple rounds of ATP hydrolysis. This apparent processivity implies that one of the functions of the cooperative interaction between the two kinesin heads in dimeric kinesin is for the reactions occurring on one kinesin head to facilitate the release of the adjacent head from the microtubule.