The development of crazes in polycarbonate is investigated with the method of ultra small angle X-ray scattering of synchrotron radiation. Measurements at T = 130 degrees C are discussed. The two-dimensional scattering patterns are analysed by means of a simple fibrillar model of the crazes. The geometrical parameters of the crazes as a function of the macroscopic draw ratio lambda(d) are determined using a curve-fitting procedure. The craze fibril volume fraction upsilon(f) shows a complex dependence on lambda(d).