A study was conducted on the effect of cotton cellulose fibres on the crystallization behaviour of isotactic polypropylene (PP) from the melt and the resulting morphology. When the PP was allowed to crystallize isothermally at 131-degrees-C, the cotton fibres acted as nucleating agents and a transcrystalline phase was created around the fibres. Quench cooling of the melt prevented the occurrence of such a phase. Transcrystalline layers of different thicknesses were created by interrupting the isothermal crystallization at certain intervals and quenching the melt. The effect of these morphologies on interfacial shear stress transfer was investigated using the single-fibre fragmentation test. It was found that the transcrystalline morphology at the fibre/matrix interface improved the shear transfer considerably when a tensile load was applied in the fibre direction. One mechanism is proposed to be particularly responsible for this increase : slow cooling favours the kinetics of the approach of PP molecules, and hence interfacial adsorption, which yields an ordered transcrystalline PP interphase having a high density of intermolecular secondary bonds with the cellulose surface. An increase in the shear transfer efficiency with increasing thickness of the transcrystalline layer was also observed.