Three Twentieth Century earthquakes that Richter assigned M≥8 and two of comparable magnitude in the Nineteenth Century imply rapid deformation within the Tien Shan. Seismic moment tensors of major earthquakes in this century suggest an average shortening rate of 7 (¿2) mm yr-1 across the Tien Shan. In the western part, however, where three of the five largest earthquakes occurred, the calculated rate is consistent with the value of ~20 mm yr-1 measured using GPS in that area by Abdrakhmatov et al. As Avouac et al. suggested, the high rate in the western part apparently is a manifestation of counter-clockwise rotation of the Tarim Basin relative to Eurasia about an axis near the east end of the Tien Shan. ¿ 2000 American Geophysical Union