Carbon fibre-reinforced epoxy resin samples made with aligned fibres, with lengths of 1 to 5 mm, had strengths and Young's moduli that were affected by fibre length, and surface condition and well as fibre volume fraction. Stress-strain trajectories were all linear, except when the fibres were coated with silicone resin. Fibre critical lengths were found to be affected by the surface condition, but composite strength could not be accounted for when the critical length was inserted into the usual equations, based on fibre slip. The results indicate that existing theoretical treatments for strength and modulus should take more account of the overstressing of fibres adjacent to fibre ends.