De novo designed peptides, capable of undergoing a thermally triggered P-strand-swapped selfassembly event leading to hydrogel formation were prepared. Strand-swapping peptide 1 (SSP1) incorporates an exchangeable beta-strand domain composed of eight residues appended to a nonexchangeable beta-hairpin domain. CD shows that, at pH 9 and temperatures less than 35 degrees C, this peptide adopts a random coil conformation, rendering it soluble in aqueous solution. On heating to 37 degrees C or greater, SSP1 adopts a beta-hairpin that displays an exchangeable fi-strand region. The exchangeable strand domain participates in swapping with the exchangeable domain of another peptide, affording a strand-swapped dimer. These dimers further assemble into fibrils that define the hydrogel. A second peptide (SSP2) containing an exchangeable strand composed of only four residues was also studied. Microscopy and scattering data show that the length of the exchangeable domain directly influences the fibril nanostructure and can be used as a design element to construct either twisted (SSP1) or nontwisted (SSP2) fibril morphologies. CD, FTIR, and WAXS confirm that both peptides adopt P-sheet secondary structure when assembled into fibrils. Fibril dimensions, as measured by TEM, AFM, and SANS indicate a fibril diameter of 6.4 nm, a height of 6.0 nm, and a pitch of 50.4 nm for the twisted SSP1 fibrils. The nontwisted SSP2 fibrils are 6.2 nm in diameter and 2.5 nm in height. Oscillatory rheology, used to measure bulk hydrogel rigidity, showed that the gel composed of the nontwisted fibrils is more mechanically rigid (517 Pa at 6 rad/s) than the gel composed of twisted fibrils (367 Pa at 6 rad/s). This work demonstrates that beta-strand-swapping can be used to fabricate biomaterials with tunable fibril nanostructure and bulk hydrogel rheological properties.