A micro-seismic field experiment has been carried out in the Marmara Sea region. The analysis of the events before and after the August 17, 1999 Izmit (Turkey) earthquake has been completed. 1446 events have been well located out of a total of 3165 recorded within the period from July 15 to November 2, 1999. 67% of the aftershocks with magnitude greater than 4 have occurred within the first 6 days after the main-shock. Earthquakes of the Izmit sequence are distributed in the first 15 km of the earth crust, but major events are located in between 5 km and 15 km depth. The seismicity pattern defines a rupture plane extending for about 150 km in an E-W direction. The rupture is extremely linear but segmented, and its complexity increases towards the western end manifesting bifurcation. A stress analysis has been carried out both at the western end and all along the aftershock zone. 96 selected aftershocks, registered between August 21 and October 22, were chosen in order to compute their focal mechanisms and obtain information about the stress regime after the Izmit earthquake. Strike-slip and normal faulting mechanisms are dominant. The numerous strike-slip mechanisms are compatible with a dextral motion on an EW oriented vertical fault plane. The best stress tensor solution shows a regime in extension with a well-defined sigma(3) axis oriented approximately N35degrees.