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The fatigue of single thermoplastic fibres has been well documented to occur in a reproducible manner when they are subjected to certain cyclic loading conditions. The fatigue fracture morphologies of these fibres are very distinctive and differ markedly from other types of failure. This type of behaviour, which is clearly seen with the unambiguous tests on single fibres, must reflect behaviour of fibres in more complex structures which are subjected to cyclic loading. Only limited numbers of reports have, however, shown similar fracture morphologies with fibres extracted from fibre bundles embedded in a matrix material such as rubber. Usually the fractured ends of fibres taken from structures are seen to be shorter than those obtained in single fibre tests and also they show more complex and confused crack growth. The present study reveals that the low thermal conductivity of the fibres, exacerbated when they are embedded in a rubber matrix, leads to very high temperature rises, which is not the case in single fibre tests and under these conditions, crack initiation occurs across the fibre section instead of being restricted to the near surface region. Tests on single fibres at temperatures up to and beyond the glass transition temperature have shown how the fracture morphologies become modified. The fatigue process has been seen to become generalised throughout the fibre and failure occurs due to the coalescence of several cracks, some of which are initiated in the core of the fibre. In all cases, the cracks can be seen to have been initiated by solid inclusions in the fibres. [ABSTRACT FROM AUTHOR]