High performance polyamide and polyester thermoplastic fibres fail by a distinct fatigue process but the microstructural phenomena involved in crack initiation, have, until now, been difficult to interpret. The process of crack initiation has been revealed as being associated with the presence of small solid inclusions, observed in all the studied fibres. The position of the inclusions is seen to be important and this may be explained in terms of fibre macroscopic morphology linked to the manufacturing process. The presence of a skin/core structure is revealed. The mechanical experiments used throughout this study were performed on single fibres with, numerous analyses being carried out using SEM, optical microscopy and birefringence techniques.