Samples of poly(trimethylhexamethylene terephthalamide) were first blended with separately prepared fumed silica in the absence of a coupling agent, and their mechanical properties were measured in continuous extension. The reinforcement obtained in this ex situ approach was qualitatively similar to that obtained by the in situ generation of silica in the sol-gel hydrolysis and condensation of tetraethoxysilane. The increase of ultimate (tensile) strength, however, was only 10% compared to the 25% obtained in the in situ approach. Adding 20 or 40 mol% of an epoxysilane couping agent to the ex situ samples gave increases in tensile strength up to 50%. Scanning electron microscopy showed interesting differences in the structures of these various fillers. For example, the fumed silica particles without the coupling agent showed a tendency to form heterogeneous aggregates in a spherical texture. The particles coated with the coupling agent showed little aggregation and evidence of good adhesion to the polyamide matrix. Concentrations up to a 20 mol% coupling agent also yielded spherical textures, but 40 mol% gave an unusual fibrous structure.