In this paper, we consider robust output regulation of distributed parameter systems with infinite-dimensional exosystems capable of generating polynomially growing signals. We design an observer-based error feedback controller solving the control problem. The controller is chosen in such a way that it incorporates an internal model of the infinite-dimensional exosystem. The remaining parameters of the controller are chosen to stabilize the closed-loop system strongly. We also analyze the classes of signals generated by the exosystem. In particular, we explore the connection between the smoothness properties of the reference and disturbance signals and the strictness of the conditions required for the existence of a controller solving the robust output regulation problem. Copyright © 2012 John Wiley & Sons, Ltd.