The authors present experimental results on endpoint position control of a single-link, very flexible robot arm carrying an unknown, varying payload. The control objective is to maintain endpoint position accuracy in the presence of flexure effects after rapid movement to a rigid-body slew-angle commanded position. Fast, simple, and efficient frequency-domain schemes are used for online controller gain adjustment within an effective scheduling framework. Only endpoint acceleration measurements and motor shaft angle measurements are utilized in relatively simple control laws, where the appropriate gains have been scheduled in accordance with modal frequency information corresponding to a varying, unknown payload.>