The exact theories advanced by Curtin(6) and Hui et al.(7) to describe the fiber break evolution process in single fiber composites are found to be incorrect when compared with experimental data. In contrast to theoretical predictions where the matrix is assumed to be elastic perfectly plastic, experimental data indicate that the sizes of the fragment lengths that survive to saturation decrease as the strain is increased. It is also shown that the break locations at saturation are uniform along the length of the fiber specimens, with the uniformity apparently being independent of interfacial shear strength, fiber type, matrix type, and fiber-fiber interactions. The theory of uniform spacings gives an explicit distribution function for the ordered fragment lengths. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 509-516, 2010