Diatom frustules were used as bio-templates to synthesize functional ceramics via solid-gas displacement reactions. Silica-based frustules were exposed to TiF4 at 330 degrees C to form TiOF2, which was later converted to TiO2 (anatase) by heat treatment in air at 600 degrees C. The TiO2 frustules were then exposed to molten Ba(OH)(2) or Sr(OH)(2) to form BaTiO3 or SrTiO3, respectively. In both cases, near-complete conversion was achieved while retaining the morphology of the original silica frustules. BaTiO3 and SrTiO3 frustules exhibit nearly phase pure, nanocrystalline perovskite structure.