The performance of biogenic opaline silica short fibers, natural (NF) or modified with vinyltrimethoxysilane (MF), as primary or secondary fillers in an elastomeric matrix of poly(dimethylsiloxane), PDMS, was evaluated in this work. Compounding was carried out on a two-roll mill, and the PDMS matrix was peroxide crosslinked by compression molding. Characterization of these fibers was performed by BET surface area, X-ray fluorescence, infrared spectrum, X-ray diffractometry, field emission scanning electron microscopy, and Bayer's test. The obtained rubbers were characterized by thermogravimetric analysis, field emission scanning electron microscopy, swelling measurements in cyclohexane, dynamic mechanical analysis, and tensile tests. As primary filler, NF and MF increased the thermal stability of the PDMS matrix, and MF also restricted the swelling of this matrix in cyclohexane. For the rubbers containing NF or MF as secondary filler, the swelling restriction was caused by both fibers. Silica short fibers were also efficient as primary filler in PDMS with regard to Young's modulus and tensile strength, and as secondary filler, NF and MF were effective reinforcing fillers in relation to storage and Young's modulus, resulting on an increase in the stiffness of the rubbers. (c) 2006 Wiley Periodicals, Inc.1011290299