A telemetered seismic network (IZINET) consisting of 13 stations has been in operation since 1993 in the western extension of the North Anatolian Fault Zone (NAFZ) in the eastern Marmara region. The network was established to study microearthquake activity in this region, which had been known as a seismic gap until the occurrence of the 17 August 1999 İzmit (Kocaeli) earthquake (Mw 7.4). For the last 7 yr, about 3250 events were recorded and located. Local magnitudes of these events, which were determined based on the duration of seismic coda waves, were generally smaller than 4.0. Seismicity in the time period studied clusters in five areas. Most of the events are found to be shallower than 20 km. It turned out that before the 17 August 1999 İzmit earthquake, the İzmit-Adapazari region was seismically more active than the other regions in the western extension of the NAFZ.