Helical oligomers of Β -peptides represent a particularly promising type of building block for directed assembly of organic nanostructures because the helical secondary structure can be designed to be very stable and because control of the Β -amino acid sequence can lead to precise patterning of chemical functional groups over the helix surfaces. In this paper, we report the use of small angle x-ray scattering measurements (SAXS) to characterize nanostructures formed by the directed assembly of Β -peptide A with sequence H2 N- Β3 hTyr- Β3 hLys- Β3 hPhe-ACHC- Β3 hPhe-ACHC- Β3 hPhe- Β3 hLys-ACHC-ACHC- Β3 hPhe- Β3 hLys-CON H2. Whereas prior cryo-TEM studies have revealed the presence of nanofibers in aqueous solutions of Β -peptide A, SAXS measurements from the nanofibers were not well-fit by a form factor model describing solid nanofibers. An improved fit to the scattering data at high q was obtained by using a form factor model describing a cylinder with a hollow center and radial polydispersity. When combined with a structure factor calculated from the polymer reference interaction site model (PRISM) theory, the scattered intensity of x-rays measured over the entire q range was well described by the model. Analysis of our SAXS data suggests a model in which individual Β -peptides assemble to form long cylindrical nanofibers with a hollow core radius of 15 Å (polydispersity of 21%) and a shell thickness of 20 Å. This model is supported by negative stain transmission electron microscopy.