The normal X-ray fibre pattern of polyacrylonitrile (PAN) consists of just two sharp equatorial peaks (d(1) similar to 0.53 nm and d(2) similar to 0.30 nm), and very diffuse meridional and off-axis scattering. This has been attributed to a hexagonal packing of chains, with no periodicity along the chain axis. In this work, we have investigated the effect of a tensile stress on this fibre pattern. It was found that both d, and d, decreased with increasing stress, but the ratio d(1)/d(2) remained constant within experimental error at root 3/1. This indicated that the ’molecular rods’ decreased in diameter on application of the stress, but their packing remained essentially hexagonal. The normal meridional scattering from PAN shows a two-component broad peak; when stress was applied, there was no shift in the positions of either component, but there was a transfer of intensity from high to low 2 theta values. This was also consistent with a slight extension of the rod. By adopting a simple model of extending rods, it was possible to compute an X-ray stress-strain curve and a modulus from the change in the two equatorial peaks.