We determined hypocenters and focal mechanism solutions of aftershocks, and three-dimensional P-wave velocity structure in and around the focal area of the 1999 İzmit earthquake (Mw 7.4). Aftershocks form a 170-km-long narrow zone trending in an east-west direction along the northern branch of the North Anatolian Fault Zone (NAFZ). They are not homogeneously distributed in the whole aftershock area, but consist of several clusters. There are three significant clusters in the aftershock activity: (1) near the hypocenter of the mainshock, (2) near 29.2° E in the Marmara Sea, and (3) east of 30.4° E. Focal mechanism solutions of aftershocks have various types. But they are similar to each other within each cluster. The P-wave velocity structure obtained has a distinct low-velocity area to the west of the mainshock hypocenter. The mainshock rupture area estimated by Yagi and Kikuchi (2000) lies outside this low-velocity area. There exists a high-velocity anomaly to the east of 30.4° E. This high-velocity area lies below the aftershock cluster distributed to the east of 30.4° E. This high-velocity anomaly extends to the shallower depth of the southern branch (Iznik-Mekece fault) of NAFZ. The Anatolian earthquake sequence that had migrated westward for the past 60 yr did not propagate into this southern branch. This suggests the possibility that the Anatolian earthquake sequence progressed to the west exploiting an area that might break more easily.