Responsive photonic crystals have been developed for chemical sensing using the variation of optical properties due to interaction with their environment(1-4). Photonic crystals with tunability in the visible or near-infrared region are of interest for controlling and processing light for active components of display, sensory or telecommunication devices. Here, we report a hydrophobic block-hydrophilic polyelectrolyte block polymer that forms a simple one-dimensional periodic lamellar structure. This results in a responsive photonic crystal that can be tuned via swelling of the hydrophilic layers by contact with a fluid reservoir. The glassy hydrophobic layer forces expansion of the hydrophilic layer along the layer normal, yielding extremely large optical tunability through changes in both layer thickness and index of refraction. Polyelectrolyte polymers are known to be highly responsive to a range of stimuli(5,6). We show very large reversible optical changes due to variation of the salt concentration of a water reservoir. These one- dimensional Bragg stacks reflect incident light from the ultraviolet-visible region to the near-infrared region (lambda(peak) = 350-1,600 nm) with over a 575% change in the position of the stop band. Our work demonstrates the extremely high responsivity possible for polyelectrolyte-based photonic materials.