Fibres from aromatic polyamides have a much higher Young's modulus than fibres from aliphatic polyamides. In order to contribute to the explanation of this observed difference we looked at one of the ultimate properties, the elastic modulus in the crystalline region in the chain direction (Ecr). We carried out measurements on a bundle of filaments of PRD 49 fibre, which we identified by i.r. and X-ray analyses as poly(p-phenylene terephthalamide). With the X-ray technique we determined the lattice extensions under loading and from these data the Ecr was calculated. The Ecr was found to be 20 × 1011 dyne/cm2 which is in good agreement with the calculated Ecr, but not very different from that of nylon-6,6. The Young's modulus was found to be 11 × 1011 dyne/cm2.