S U M M A R YDifferential radar interferometry provided high-quality near-field deformation data for the 2003 Bam earthquake and therefore strong constraints on its source parameters. The ruptured fault segments could be clearly detected by using a Sobel Edge Filter on the phase-unwrapped deformation field. The estimated total rupture length is about 24 km. More than 80 per cent of the seismic moment was released from its southern segment of about 13 km, where the slip reached a maximum of up to 270 cm resulting in a stress drop of at least 6 MPa. In addition, optical remote sensing data show that the Bam fault is not a single fault but consists of a 4-5 km wide fault system with the known main branch running between the city of Bam and Baravat. The fault ruptured by the Bam earthquake appears to continue the NW branch of this fault system from Bam city southwards. Based on these results, we suggest that the Bam earthquake ruptured a hidden or new fault and that in this process an unusually strong asperity was involved.