The use of a biomolecular RNA template for the synthesis of CdS semiconductor nanocrystals is described. Transfer RNA, with a well-defined three-dimensional structure, is used as a scaffold and ligand system in the aqueous synthesis of CdS. We have found that nanocrystal structures can be modulated by the structure of the templating tRNA. When possessing a folded three-dimensional structure, tRNA-templated synthesis yields a single product. If the same tRNA is rendered unstructured through the introduction of destabilizing mutations, a range of products are observed. The results reported indicate that biomolecules can be used to systematically engineer the structures and properties of semiconductor-based materials, and that the synergy between the dimensions of nanostructures and biomolecules provides a means to tune the properties of materials with nanoscale precision.