Control of fluid rheology by light (i.e. photorheological or PR fluids) has been a subject of significant attention with the view to apply it in a variety of applications, such as in sensors, dampers, and valves for microfluidic or MEMS devices. Recently (JACS 129, 1553 (2007)), we reported a cationic surfactant-based fluid whose viscosity can be rapidly decreased by UV irradiation (i.e., photothinning). Here, we describe a formulation based on a zwitterionic surfactant and a photosensitive cinnamic acid derivative that shows a rapid increase in viscosity upon exposure to UV radiation (i.e., photogelling). At first, the surfactant/photo-additive mixture in water has a low viscosity, indicating the presence of small micelles. Upon UV irradiation, the trans form of the cinnamic acid derivative is converted to the cis form. This molecular geometry change alters the molecular packing of the surfactant/photo-additive complex, which in turn, induces the growth of long wormlike micelles. Thus the sample is transformed from a thin, runny fluid with a viscosity close to water to a highly viscoelastic, gel-like fluid. We will present evidence from UV-Vis spectroscopy, small-angle neutron scattering (SANS), rheological, and zeta-potential studies, that will systematically reveal the molecular and microstructural mechanism for our results.