A vibration suppression strategy is developed for a flexible manipulator with a collocated piezoelectric sensor/actuator pair. A control law is developed based upon positive position feedback and is augmented with an adaptive parameter estimator based on the recursive least squares method to update the first two natural frequencies of the structure online. For the positive position feedback control law, accurate targeting of the modes is critical for vibration control. Experiments are then conducted to show that the controller can be used to suppress the vibrations of a structure with unknown natural frequencies.