The non-hookean stress-strain response of carbon fibres was investigated in relation to changes in crystallite orientation with tensile stress. Various one-dimensional array models and a mosaic model were examined. Amongst these models, only the mosaic model in which the stress of the crystallites can be transmitted in both the transverse and the axial directions showed any quantitative agreement with the measured increases in the tensile modulus and the crystallite orientation with tensile stress. This suggests that deformation of the crystallites is constrained with increasing tensile stress. It was also found that the ratio of the tensile stress of the fibre to that of the crystallites is close to the crystallite volume fraction rather than the ratio of the fibre density to the crystallite density.