The ability of an aqueous extract of W. somnifera L. Dunal (Family: Solanaceae) roots to inhibit fibril formation by the amyloid-β peptide in vitro was investigated. W. somnifera is used extensively in traditional Ayurvedic medicine as a nerve tonic with reputed memory enhancing properties. Inhibition of fibrillogenesis measured by transmission electron microscopy and ThT fluorescence assay showed that an aqueous extract of W. somnifera strongly inhibited Aβ fibril formation in a concentration-dependent manner, when compared with control samples. These results suggest that the aqueous extract of W. somnifera root has an ability to inhibit the formation of mature amyloid-β fibrils in vitro, which are known to lead to amyloid plaque formation in vivo. Copyright © 2011 John Wiley & Sons, Ltd.