We describe the novel structure and behavior of a DNA-DDAB complex film cast from an organic solvent and exhibiting a structural switching transition as it is dried or wetted with water. The film was easily prepared by formation of a complex between the negatively charged phosphate groups of DNA and the positively charged headgroup of the surfactant DDAB. This complex was then purified, dried, dissolved in 2-propanol, and cast onto a glass slide to form a self-standing film by means of slow evaporation. While the structure of the dried film was found to be composed of single-stranded DNA and a monolayer of DDAB, upon hydration of the film, the structure switched to double-stranded DNA complexed to a bilayer of DDAB. We expect this phenomenon to serve as a useful model for the design of new responsive materials and programmable self-assembly.