The crystalline structure and fibrillar texture of nylon-6 fibers filled with nanosized particles were investigated using wide-angle and small-angle X-ray scattering. As-spun fibers filled with organic nanoparticles consisting of aromatic polyamide-like hyperbranched molecules with amine-terminating groups exhibited strong modification of both the molecular orientation and the crystalline structure compared with that of unfilled spun fibers. Montmorillonite-filled fibers mainly exhibited orientation improvement. The differences are discussed in terms of the rheological and nucleating effects during spinning. Drawing at 140 degreesC involves structural changes that resulted in the three kinds of fibers having a similar crystalline form and molecular orientation. In parallel, after significant strain-induced changes, the microfibrillar texture of the various fibers displayed subtle differences at the ultimate stage of drawing. The changes in the fibril long period and fibril radius as a function of draw ratio are discussed in terms of the two sequential deformation processes of microfibril stretching and microfibril slipping. The occurrence of interfibrillar strain-induced cavitation is discussed in relation to the nature of the interactions between the filler and the nylon-6 matrix. And, finally, the mechanical properties are discussed in relation to the filler-matrix interaction. (C) 2004 Wiley Periodicals, Inc.