Flax provides a renewable source of fibres with good mechanical properties, which justifies its use as a reinforcement for polymers. Composite materials made with flax which has only undergone mechanical treatments (stripping followed by combing) results in the presence of bundles of fibres. When a unidirectional flax/polyester ply is loaded in transverse tension the damage mechanisms are complex; cracks develop in the matrix, and at the fibre/matrix interface, but they also appear within the fibres themselves and in the lamellae within the fibre bundles. This emphasises the highly anisotropic nature of these flax fibres, whose average longitudinal Young's modulus is 59 GPa and whose transverse modulus is estimated here to be 8 GPa. (c) 2006 Elsevier B.V. All rights reserved.