Deformation in the boundary zone between the Anatolian and Eurasian plates has been determined by means of repeated Global Positioning System (GPS) measurements. Three campaigns (1990, 1992, 1994) were carried out around the Marmara Sea. The network lies within the active earthquake belt of NW Anatolia, extending from the western part of the North Anatolian Fault Zone (NAFZ) to the Aegean extensional province. Results of the dense network consisting of 52 sites clearly reveal the westward motion of Anatolia. The rates amount to 17 mm/a¿3 mm/a on average relative to Istanbul, located at the north-eastern edge of the Sea of Marmara. Based on the trajectories for the displacements of the observation sites it is concluded that the Marmara Sea forms a transition zone between the Europe fixed region of Thrace/Black Sea (Eurasia) and NW Anatolia. At the western extremity of NW Anatolia the motion bends from W to WSW(247¿). The strain field calculated yields an E-W oriented right-lateral strike-slip movement (average 0.12 ppm/a, maximum 0.41 ppm/a) accompanied by NE-SW extension (average -0.12 ppm/a, maximum -0.52 ppm/a) at the eastern end of the Marmara Sea and N-S extension at the Aegean shore line. Comparisons with the directions of T axes derived from 65 fault plane solutions of earthquakes show remarkable agreement. ¿ American Geophysical Union 1995.