A new class of light-responsive viscoelastic fluids based on anionic wormlike micelles is reported. The key components are sodium oleate (NaOA) and a cationic azobenzene dye, 1-[2-(4-phenylazo-phenoxy)ethyl]-3-methylimidazolium bromide (C(0)AZOC(2)IMB). These binary systems are gel-like fluids at certain concentration ratios of [C(0)AZOC(2)IMB]/[NaOA], e.g. 35/100, owing to the formation of long, entangled wormlike micelles. The viscosity of these fluids can be controlled reversibly by light due to photo isomerization between trans-C(0)AZOC(2)IMB and cis-C(0)AZOC(2)IMB. For example, the zero-shear viscosity (eta(0)) of an originally gel-like sample is high up to similar to 1300 Pa s when C(0)AZOC(2)IMB is in its trans from, whereas the mixture becomes a Newtonian fluid with no about 0.01 Pa s after UV light irradiation. For the post-irradiated cis-C(0)AZOC(2)IMB, short cylindrical micelles form, hence accounting for the lower viscosity. Evidence for the structural transition is provided by UV-vis spectra, rheology, H-1 NMR and cryo-transmission electronic microscopy measurements. (C) 2013 Elsevier Inc. All rights reserved.