In nature, peptides bind metals reproducibly and selectively via specific amino acid sequences, producing nanocrystals in controlled sizes and morphologies. A novel method of growing uniform nanocrystals directly on biological nanotubes by immobilizing synthetic Ni-mineralizing peptides on the nanotubes is reported (see Figure). Control of the Ni nanocrystal size is a function of pH, enabling tunability of magnetic properties in the resulting nanotube.