A network model is applied to describe the deformation behaviour of melt-spun Nylon-6 fibres obtained at different spinning and drawing conditions. The network draw ratio is determined from analysis of true stress-strain curves and correlated with fibre orientation. From this analysis it appears that the deformation behaviour of Nylon-6 fibres adheres to a pseudo-affine mechanism. The results are discussed in terms of the semicrystalline nature of Nylon-6 fibres. By means of on-line birefringence measurements, wide angle X-ray scattering (WAXS) and solid-state NMR, it is shown that the crystalline and amorphous phases in Nylon-6 respond in a different way to network deformation. In the spin line, an affine (rubber-like) deformation mechanism is observed and this rubber-like network behaviour is transferred to the amorphous phase. After winding and conditioning of the spun fibres, a crystalline phase with relatively high orientation is formed which deforms according to a pseudo-affine mechanism upon further stretching. (C) 2003 Elsevier Ltd. All rights reserved.