One of the most promising routes for exploiting the axial properties of carbon nanotubes (CNTs) is in a macroscopic fibre consisting of CNTs aligned parallel to each other and to the fibre axis. In this work, we study the structure of and stress transfer in CNT fibres produced by direct spinning from the gas-phase during CNT growth. Using Raman spectroscopy, X-ray diffraction (XRD) and high-resolution electron microscopy we show that the fibres are mostly composed of collapsed double-wall nanotube stacks highly aligned and uniformly oriented along the fibre axis. In situ Raman and XRD measurements during tensile deformation indicate that the transfer of stress between bundles in the fibre is not uniform, with the Raman peak downshift rates for the D, G and G′ bands varying by as much as a factor of 2 in different areas of the fibre. The infiltration of polymer into the CNT fibre increases stress transfer, the highest value of the G′ band downshift rate observed being -33.6 cm -1/% fibre strain; however, with a non-uniform stress distribution still observed after polymer ingress. © 2011 Elsevier Ltd. All rights reserved.