The thermal residual stress induced in the fiber during specimen preparation, prior to a single-fiber fragmentation experiment, is studied using various theoretical schemes. It is found that the longitudinal component of this stress is compressive in nature, and that in some cases it is high enough to induce breaks in the fiber. The number of such pre-existing breaks may be calculated under certain conditions, one of them being that the Weibull shape parameter of the fiber compressive strength distribution be known. Experimental data produced with high-modulus graphite/polypropylene single-fiber composites are used to substantiate the arguments presented. Implications for high fiber content composites are discussed.