A temporary seismic network installed in the eastern part of the aftershock region of the 1999 İzmit, Turkey, earthquake revealed that aftershock epicenters were in a very linear alignment near the eastern end of the earthquake fault. The linear alignment extended over a length of about 15 km along the segment of the earthquake fault in an ENE-WSW direction. Hypocenters on the segment are confined to a narrow depth range near the bottom of the seismogenic region. It is inferred from these results that the afterslips occurred in the seismogenic region on the segment before the 1999 Düzce earthquake that occurred in the eastern extension of the İzmit earthquake fault nearly 3 months after the İzmit earthquake. It is possible that the afterslips concentrated larger stress on the fault of the 1999 Düzce earthquake and generated the earthquake earlier.