A　new　gemini　surfactant　with　an　azobenzene　group　incorporated　in　the　spacer　has　been　synthesized.　In　water,　this　new　compound　forms　a　photo-responsive　fluid,　and　the　properties　have　been　characterized　by　rheology　and　freeze-fracture　electron　microscopy　(FF-TEM).　UV/Vis　irradiation　causes　a　reversible　change　in　zero　shear　viscosity　of　about　5　orders　of　magnitude.　The　different　rheological　properties　correspond　to　different　aggregation　states　in　solution.　A　densely　entangled　network　and　small　discrete　micelles　are　respectively　responsible　for　the　high　viscosity　and　the　low　viscosity　state,　as　revealed　by　FF-TEM.　The　simplicity　and　high　photo-efficiency　of　this　single-component　system　act　as　a　foundation　for　designing　novel　stimuli-responsive　materials.　©　2009　Elsevier　Inc.　All　rights　reserved.