The GPS velocity field of the Aegean Sea and western Anatolia is used to determine crustal deformation strain rates for the period 1988 to 1996. The zone from the Marmara Sea to the North Aegean Trough is associated with strong right-lateral shear motion, with maximum strain rates of 170 nstrain/a. In the northern Aegean Sea the extensional deviatoric axes are oriented NNE-SSW, associated with strain rates reaching 150 nstrain/a, and in western Anatolia varying around N-S with rates of up to 95 nstrain/a. The extensional areas mostly coincide with active graben features. The south-western Aegean Sea is almost strain-free. In general, the areas of high geodetic strain rates are accompanied by distinct seismic clusters whereas the strain-free regions are nearly aseismic. ¿ 1999 American Geophysical Union