A range of multi-wall carbon nanotubes and carbon nanofibres were mixed with a polyamide-12 matrix using a twin-screw microextruder, and the resulting blends spun to produce a series of reinforced polymer fibres. The aim was to compare the dispersion and resulting mechanical properties achieved for nanotubes produced by the electric arc and a variety of chemical vapour deposition techniques. A high quality of dispersion was achieved for all the catalytically-grown materials and the greatest improvements in stiffness were observed using aligned, substrate-grown, carbon nanotubes. The use of entangled multi-wall carbon nanotubes led to the most pronounced increase in yield stress, most likely as result of increased constraint of the polymer matrix due to their relatively high surface area. The degrees of polymer and nanofiller alignment and the morphology of the polymer matrix were assessed using X-ray diffraction and differential scanning calorimetry. The carbon nanotubes were found to act as nucleation sites under slow cooling conditions, the effect scaling with effective surface area. Nevertheless, no significant variations in polymer morphology as a function of nanoscale filler type and loading fraction were observed under the melt spinning conditions applied. A simple rule-of-mixture evaluation of the nanocomposite stiffness revealed a higher effective modulus for the multi-wall carbon nanotubes compared to the carbon nanofibres, as a result of improved graphitic crystallinity. In addition, this approach allowed a general comparison of the effective nanotube modulus with those of nanoclays as well as common short glass and carbon fibre fillers in melt-blended polyamide composites. The experimental results further highlight the fact that the intrinsic crystalline quality, as well as the straightness of the embedded nanotubes, are significant factors influencing the reinforcement capability. (C) 2004 Published by Elsevier Ltd.