Background: KIF3AB is a heterotrimeric plus-end-directed kinesin-2 motor, implicated in intraflagellar transport. Results: KIF3AB shows rate-limiting ADP release upon microtubule collision and an unusual apparent weak ATP affinity observed at microtubule⅐KIF3AB dissociation. Conclusion:The presteady-state kinetics suggest a novel ATPase mechanism for KIF3AB stepping. Significance: The KIF3AB kinetics reveal the mechanistic diversity that kinesin motors exhibit for cargo transport.