Enzymes and other biomolecules are often immobilized in a matrix to improve their stability or to improve their ability to be reused. Performing a polycondensation reaction in the presence of a biomolecule of interest relies on random entrapment events during polymerization and may not ensure efficient, homogeneous, or complete biomolecule encapsulation. To overcome these limitations, we have developed a method of incorporating autosilification activity into proteins without affecting enzymatic functionality. The unmodified R5 silaffin peptide from Cylindrotheca fusiformis is capable of initiating silica polycondensation in vitro at ambient temperatures and pressures in aqueous solution. In this study, translational fusion proteins between R5 and various functional proteins (phosphodiesterase, organophosphate hydrolase, and green fluorescent protein) were produced in Escherichia coli. Each of the fusion proteins initiated silica polycondensation, and enzymatic activity (or fluorescence) was retained in the resulting silica spheres. Under certain circumstances, the enzymatically-active biosilica displayed improved stability relative to free enzyme at elevated temperatures. (C) 2009 American Institute of Chemical Engineers Biotechnol. Prog., 25: 417-423, 2009