The fiber fragmentation phenomenon in the single‐filament composite test is currently among the most intensively researched aspects of composite micromechanics. By this method the interfacial shear strength, a physical parameter which reflects the quality of the fiber/matrix bonding, is calculated. In most studies various statistical models for the distribution of fragment lengths have been assumed without any justification other than a good fit to experimental data. Here we argue that if the flaws are assumed to be distributed along the fiber length according to a spatial Poisson process, then far from the saturation point the resulting fragment lengths must exactly follow a shifted exponential distribution. At the saturation limit, the distribution of fragment lengths is still approximately exponential. This is confirmed by single‐filament composite experiments with high strength carbon fibers embedded in epoxy. Cumulative distributions of interdefect spacings at various levels of stress (or strain) are made available by means of an experimental technique described here. It is shown that the resulting data may be used to unfold the average strength versus length dependence in the single fiber, and to calculate the relevant Weibull shape parameter.