Melt-spun fibers produced from poly(butylene terephthalate) and containing very small amounts of multiwall carbon nanotubes (MWNTs) have been investigated. Preliminary studies have been carried out on fibers produced at low draw ratio. The additive contents (<1% by weight) and process conditions for the production of fibers have been optimized to obtain a satisfactory level of MWNTs dispersion for all systems, as confirmed by electron microscopy observations. Structural investigations performed on fibers with varying content and aspect ratio of nanotubes made it possible to relate the observed changes in properties, such as dynamic mechanical behavior and mechanical properties, to the crystalline structure of the polymer in terms of size, main distance between the crystalline lamellae, and orientation. The results have revealed unexpectedly large increase in elongation at break-in fibres containing MWNTs, even though these exhibited a slight increase in the stiffness of the fibers. This effect has been attributed to structural changes and has also been found to affect the nanotubes' external diameter. (C) 2011 Wiley Periodicals, Inc. Adv Polym Techn 30: 41-50, 2011; View this article online at wileyonlinelibrary.com. DOT 10.1002/adv.20206