We have investigated the spatial variation of the stress field along the fault rupture zone of the 1999 Izmit earthquake (M-W 7.4) using first-motion polarity data at seven distinct aftershock clusters. In our approach, the first-motion polarities of all the aftershocks in a cluster are simultaneously inverted to determine the stress tensor parameters and fault plane solutions of individual events, in accordance with the method of Horiuchi et al. (1995). Where post-seismic slip was significant (e.g., Sapanca, Sakarya-Akyazi, and Karadere segments), we obtained stress tensors with the fault parallel or fault normal maximum (sigma(1)) and minimum (sigma(3)) principal compressive stress axes, which may imply either low frictional coefficients or fault weakness. A stress tensor with similar features was derived from the Cinarcik cluster, where the aftershocks lie in a low-velocity zone beneath the geothermal area. The maximum principal stress axis tends to remain parallel to the trend of the pre-mainshock sigma(1) around the Yalova segment; this segment experienced little to no co-seismic displacements. The stress tensor around the Golcuk segment, where the largest surface displacement of 5.5 m was observed, was determined to be 20-25 degrees counterclockwise rotated, but the aftershock alignment remained fault parallel. We interpret these results in terms of the strong crust. On the other hand, both the aftershock alignment and the stress tensor were found to be rotated in the Izmit earthquake epicentral region despite the lower co-seismic displacements. We attribute this feature to the weak crust.