In this paper, an add-on multirate adaptive control scheme is proposed to compensate for uncertain mechanical resonant modes beyond the Nyquist frequency in mechatronic systems. The multirate adaptive controller is incorporated into the existing sampled-data control system sampled at a slow rate. To integrate well with the pre-designed feedback controller, an auxiliary error instead of the regulation error is utilized to design the multirate adaptive controller. By using a polynomial transformation technique and a recursive least-squares algorithm with a dead-zone, the proposed multirate adaptive control ensures boundedness of all the closed-loop signals and convergence of the auxiliary error. Our simulation and experimental results on the head-positioning servo control system of a hard disk drive demonstrate the effectiveness of our proposed compensation scheme in attenuating the vibration for uncertain resonances beyond the Nyquist frequency. (C) 2015 Elsevier Ltd. All rights reserved.