A series of poly(trimethylene terephthalate) (PTT) nanocomposites, containing an organically modified montmorillonite (C12PPh-MMT), were prepared by in situ intercalation polymerization of dimethyl terephthalate (DMT) and 1,3-propanediol (PDO). The PTT nanocomposites were melt-spun at different organoclay contents and different draw ratios (DRs) to produce monofilaments. The nanocomposites were characterized by X-ray diffraction, electron microscopy, universal tensile testing, differential scanning calorimetry and thermogravimetric analysis. Some of the clay particles appeared well dispersed within the PTT matrix, while others were found to agglomerate with a size greater than 10 nm. The addition of a small amount of C12PPh-MMT was sufficient to improve the thermomechanical properties of the PTT hybrid fibers. Both the thermal stability and the tensile strength increased with increasing clay content at DR = 1. As the DR was increased from 1 to 9, the ultimate tensile strength of the hybrid fibers decreased, while the initial modulus remained constant. (C) 2004 Elsevier Ltd. All rights reserved.