This paper deals with the development of a new robust fractional-order PI controller for hydraulic canals whose parameters can vary in a wide range. Robustness to gain changes has particularly been increased. A method to design these controllers through the use of frequency specifications is proposed. Analysis of the frequency response of the closed-loop canal shows that the general robustness was also enhanced. Two of these fractional controllers have been experimentally implemented in a laboratory hydraulic canal characterized by time-varying dynamical parameters. A comparison between the real-time responses of the fractional-order control system and those of the standard PI demonstrates the effectiveness of the proposed fractional-order control strategy in terms of performance and robustness. © 2009 ASCE.