Test frequency exerts a considerable influence on dynamic mechanical behaviour of carbon fibre reinforced materials. In this study the effect of test frequency on the fatigue behaviour of the T300/914C system, a carbon fibre/epoxy matrix composite, is examined. Tension-tension fatigue tests were carried out at various stress levels and at three test frequencies (5, 10 and 20 Hz) for three specimen orientations (unidirectional, (0)(8), crossply (0 degrees /90 degrees)(4s) and angleply (+/- 45 degreesc)(4s)). A number of dynamic mechanical properties were monitored throughout specimen lifetimes and subsequently analysed - stress/life behaviour, maximum strain, normalised fatigue modulus, dynamic loss modulus, damping factor and specimen temperature. Frequency effects are found to profoundly influence the fatigue behaviour of both crossply and angle ply specimens. Angleply specimen fatigue response exhibits a strong dependence on test frequency, a fact that is reflected in the dynamic mechanical property responses monitored. The data obtained for unidirectional specimens is inconclusive due to the large degree of fatigue scatter observed.