The enzymatic cleavage of a peptide amphiphile (PA) is investigated. The self-assembly of the cleaved products is distinct from that of the PA substrate. The PA C-16-KKFFVLK is cleaved by alpha-chymotrypsin at two sites leading to products C-16-KKF with FVLK and C-16-KKFF with VLK. The PA C-16-KKFFVLK forms nanotubes and helical ribbons at room temperature. Both PAs C-16-KKF and C-16-KKFF corresponding to cleavage products instead self-assemble into 5-6 nm diameter spherical micelles, while peptides FVLK and VLK do not adopt well-defined aggregate structures. The secondary structures of the PAs and peptides are examined by FTIR and circular dichroism spectroscopy and X-ray diffraction. Only C-16-KKFFVLK shows substantial beta-sheet secondary structure, consistent with its self-assembly into extended aggregates, based on PA layers containing hydrogen-bonded peptide headgroups. This PA also exhibits a thermoreversible transition to twisted tapes on heating.