Crust and upper mantle S-wave velocity structure has been inferred for several distinct portions of the Gulf of California, Baja California, and Sonora, Mexico using seismic surface-wave dispersion measurements. Earthquakes in the Gulf and seismograph stations in the northern gulf, southern California, and Arizona have been used here. Fundamental-mode Rayleigh-wave data from 10- to 40-sec periods and first-higher-mode waves from 6- to 10-sec periods are used to delineate structure for five distinct sets of paths in and adjacent to the gulf. Baja California and Sonora (mainland Mexico) have similar crustal thicknesses of about 25 km. Because of interference between the fundamental and higher modes, Love-wave group velocities for Baja paths are unusually high with respect to values expected on the basis of Rayleigh-wave dispersion and were not used to determine structure. Short-period higher-mode Rayleigh waves provide a particularly good constraint on the average crustal thickness beneath Baja. For paths within the Gulf of California, there is considerable variation in the observed Rayleigh-wave dispersion. Although lateral-refraction effects may be influencing the dispersion results, consistency of observations for similar paths suggests the presence of segments of the gulf with distinct structures and permits some estimates to be made of local variations in crustal structure normal to the axis of the gulf. In the deepest portions of the gulf, average structure is near-oceanic with a crustal thickness of 9 km, but is near 20 km in the central and northeastern gulf.