It was shown that sepiolite fibers impart to natural rubber (NR) a higher degree of reinforcement than the spherical silica particles on account of their high aspect ratio. In styrene–butadiene rubber, a comparable reinforcement is obtained for both silica and sepiolite filled composites. One reason may be that sepiolite has less interaction with SBR than with NR which causes a poor dispersion and consequently reduced interaction between the polymer and the filler. The presence of several agglomerates is revealed by transmission electron microscopy. On the other hand, silica used with Si69 produces a more significant reinforcement effect than that obtained with in situ generated particles which were growing within the preformed elastomeric network after hydrolysis of tetraethoxysilane used to swell the polymer. Moreover,the use of the coupling agent in this study, is expected to induce polymer–filler bonding that increases the modulus.