The properties of fibre-reinforced composites are dependent not on ly on the strength of the reinforcement fibre but also the distribution of fibre strength. In this study, the single filament strength of several lots of Nextel(TM) 610 and Nextel(TM) 720 ceramic fibres was measured. Fracture statistics were correlated with the effects of gauge length and diameter variation, and the Weibull modulus was calculated using several different techniques. It was found that the measured Weibull modulus at a single gauge length did not accurately predict either the gauge length or diameter dependence of tensile strength.