The effect of an azobenzene-based photoresponsive surfactant on fibril formation of beta-amyloid (1-40) (A beta 40) has been studied using small-angle neutron scattering (SANS), atomic force microscopy (AFM), and light scattering (LS) measurements. Fibril formation is inhibited with a Jag phase persisting for approximately 5 h in the presence of the trans isomer of the photosurfactant under visible light (i.e., the relatively hydrophobic, activated form). Conversely, only a 2-h lag phase is observed under UV light with the cis photosurfactant isomer (relatively hydrophilic, passive form), while large fibril networks are immediately observed for the pure protein. Furthermore, in situ UV illumination of a solution of trans surfactant and protein results in rapid fibril formation. Thus, the ability to photoreversibly inhibit and trigger the fibrilization process with light illumination is demonstrated. Shape-reconstruction analysis of the SANS data is used to obtain novel information on the conformation of the protein during the initial stages of protein aggregation. Small, cylindrical protein aggregates 5 nm in diameter and 7 nm long are initially observed during the lag phase independent of the sample conditions. AFM images confirm both the aggregate structure and the duration of the lag phase and further suggest that these early aggregates appear to be the nuclei for longer aggregates that develop over time.