Unidirectional glass and carbon fibre reinforced Nylon 6 composites have been prepared by the in situ anionic polymerization of caprolactam on the fibres. In this way, it is possible to control the solidification and crystallization characteristics of the nylon. The morphology of the nylon around the fibres has been determined using sections of 0.15 Vf(volume fraction) composites and also thin films containing single fibres. The fibres nucleate directional crystallization (columnar growth) of the nylon around the fibres. The amount and form of the columnar growth depends on the type of fibre and the polymerization conditions. Preliminary studies on the mechanical properties and fracture characteristics of 0.15 Vf composites have been made and the results are discussed in terms of the matrix and interface morphology.