In the present paper, the interfacial properties of extruded and compression moulded natural fibre reinforced thermoplastics (NFRTP) are studied. The interfacial shear strength of PE-sisal composites was measured using the single fibre fragmentation test (SFFT). The difficulties in obtaining significant measurements are discussed and assessed. The main problems found were the non-transparent nature of the matrix, the irregular shape of the fibres, and the variability in fibre-matrix adhesion encountered even in single fibre specimens. Scanning Electron Microscope (SEM) pictures obtained from fractured surfaces were used for a qualitative evaluation of the interfacial properties of NFRTPs. Fibre treatment with stearic acid increased the interfacial shear strength by 23% with respect to untreated fibres. The improvements in interfacial shear strength found for the treated specimens were consistent with observations from SEM fractographs. © 2005 Elsevier Ltd. All rights reserved.