With their unique ability for substrate recognition and their sequence-specific self-assembly properties, peptides play an important role in controlling the mineralization of inorganic materials in natural systems and in controlling the assembly of soft materials into complex structures required for biological functions. Here we report the use of an engineered heptapeptide that can differentiate between the crystalline anhydrous polymorphs of calcium carbonate. This peptide contains the positively charged amino acid arginine as well as proline rather than the prototypical negatively charged aspartate or glutamate units. Its affinity to vaterite compared to aragonite was demonstrated by fluorescence microscopy using biotinylated peptides. Crystallization experiments in the presence of the vaterite-affine peptide afforded only vaterite, whereas a mutant peptide, where a proline residue was replaced by glycine, exclusively leads to the formation of calcite.