Fluids with photoresponsive rheological properties can be useful in a variety of applications, such as in sensors, dampers, and valves for microfluidic or MEMS devices. Currently, such fluid formulations are available only to a few research groups since they tend to be based on specialized photosensitive molecules (e.g. a photosensitive surfactant or polymer). Here, we describe formulations based on a commercially available zwitterionic surfactant and a photosensitive cinnamic acid derivative. These fluids exhibit a rapid and controllable increase in viscosity (gelling) upon exposure to UV radiation. Initially, the fluid has a low viscosity, indicating the presence of small micelles. Upon irradiation in the UV range, the cinnamic acid derivative undergoes a trans to cis photoisomerization. This change in geometry alters the molecular packing of surfactant/acid complex, transforming spherical micelles into long wormlike micelles. In turn, the solution viscosity is increased by more than five orders of magnitude. Small-angle neutron scattering (SANS) is used to confirm the dramatic increase in micellar length.