For a propagting crack under cyclic loading conditions the behaviour of the craze at the crack tip in an amorphous transparent polymer has been investigated. A special apparatus has been built to apply a cyclic load up to 30 N at a frequency up to 250 Hz on the cracked sample. The experimental set-up allows the simultaneous measurement of crack speed, the instantaneous load on the sample and the microscopic interference pattern of the craze at any moment of the cycle.Fracture toughness and craze length as functions of crack speed and fatigue crack growth rate have been measured on PMMA in wide frequency range.By means of the Dugdale model describing the craze shape at the crack tip, aparticular material property could be derived: the lifetime (number of loading cycles) of a libril in the craze as a function of the local stress applied on the craze