Electron microfractography has been used to study ductile fracture in maraging steel. The conditions under which the cavities, responsible for this type of fracture, can be initiated at precipitates formed during the aging treatment at 475 and 525 C (890 and 980 F),have been investigated. To this end, the microstructure has been examined as a function of aging time, and the mechanical properties and fracture appearance have been studied for different aging times and test temperatures. The fracture surface is always dimpled: for relatively short aging times the dimples are large and irregular in size; in the overaged condition the fracture surface consists of very fine dimples, and there is a good correlation between precipitates and dimples. It is concluded that cavities can be initiated on precipitates only if these particles are larger than a critical size. Qualitatively, similar results have been obtained for other precipitation hardened alloys. The critical size decreases as the test temperature is lowered. It may also depend on the shape of the particles and other factors such as the nature of the precipitates and the particle-matrix interface. Certain aspects of the changes in mechanical properties with heat treatment can be explained in terms of the existence of a critical particle size for cavity nucleation.