Interfacial strength of Ti beta 21S/SCS 6 composite was increased via ageing. Composite specimens with a(o)/W ratios of 0.24 and 0.07 were prefatigued in three point bending at ambient and at 500 degrees C both in the air and vacuum. Bridging fibres were extracted from the pre-fatigued specimens and tested in tensile loading to monitor the strength degradation due repeated opening and closing of the fatigue crack. A tri-modal Weibull distribution was employed to express strength distribution of the SCS 6 fibres. Results have shown that cyclic loading of the composites lower the mean tensile strength of the fibres by 20%, compared to the unfatigued composite. A marked effect of initial unbridged defect size was observed, when the tensile strength data were divided into two sub-population using the fatigue crack length values. Compared to unfatigued composite, tensile strength reductions of 41 and 22% were measured from the prefatigued composites with a(o)/W of 0.24 and 0.07, respectively.