A large variety of functional self-assembled supramolecular nanostructures have been reported over recent decades'. The experimental approach to these systems initially focused on the design of molecules with specific interactions that lead to discrete geometric structures(1-4), and more recently on the kinetics and mechanistic pathways of self-assembly(5,6). However, there remains a major gap in our understanding of the internal conformational dynamics of these systems and of the links between their dynamics and function. Molecular dynamics simulations have yielded information on the molecular fluctuations of supramolecular assemblies(5-7), yet experimentally it has been difficult to obtain analogous data with subnanometre spatial resolution. Using site-directed spin labelling and electron paramagnetic resonance spectroscopy, we measured the conformational dynamics of a self-assembled nanofibre in water through its 6.7 nm cross-section. Our measurements provide unique insight for the design of supramolecular functional materials.