The development of crazes in polycarbonate was investigated with the method of ultra-small-angle X-ray scattering (USAXS) of synchrotron radiation. Measurements at different temperatures and with different draw rates were carried out. The two-dimensional scattering patterns were analyzed by means of a fibrillar model of the craze. The geometrical parameters of the craze as a function of the macroscopic draw ratio were determined by using a curve-fitting procedure. From the measured values of the diameter and the mean distance of the fibrils, it is possible to calculate the volume fractionvfof the fibrils directly. Additional scattering caused by submicrocracks is discussed.