The mechanisms of crack initiation in tensile fatigue of single-edge notched specimens of polycarbonate of varying thickness have been elucidated. At low stresses and long times microcracking and localized yielding occurred to form regular diamond-shaped cells on a scale of 2-4 mu m. On increasing the stress level with thin specimens (< 1 mm), the microshear bands coalesced to form macroscopic damage zones of yielded material around the notch, followed by crack tearing from the notch surface. With increasing specimen thickness, restriction of shear banding ensued and a stable, semi-elliptical cavitation, or pop-in, formed about 10-100 mu m ahead of the notch, dependent on specimen geometry. As a result, the ligament formed between the notch and pop-in consists of yielded material. Brittle behaviour resulted with further increases in specimen thickness on loading, i.e. when the ligament could not be stabilized.