We use Global Positioning System (GPS) measurements to make the first geodetic study of the Semidi segment of the Alaska-Aleutian subduction zone. This segment, which sustained an MW8.2 earthquake in 1938, lies between Kodiak Island where the subduction interface appears to presently be fully locked, and the Shumagin Islands segment where substantial aseismic slip occurs. We invert the GPS station velocity estimates using a nonlinear least squares algorithm to solve for the width of the locked zone, the dip, and the interseismic coupling of a model subduction interface. The data are consistent with a shallow plate interface dipping ~6¿, a locking depth of ~23 km, and high interseismic coupling of ~80%. ¿ 2001 American Geophysical Union