Global Positioning System (GPS) geodetic measurements of 350--650 km baselines across the Pacific-North America plate boundary in the Gulf of California are presented. The analysis employs a four-station U.S. fiducial network and combined carrier phase and pseudorange data. Water vapor radiometer (WVR) data at the Gulf sites are used to calibrate the GPS signal for wet tropospheric path delays. Residual tropospheric delays are modeled as first-order exponentially correlated stochastic processes. The measurement precision for horizontal components is a few parts in 108 or better. Comparison of the Gulf data with other geodetic techniques is not yet possible, however the system accuracy appears to be about 1 part in 107 based on simultaneous GPS solutions of a baseline in California collocated with VLBI. The results are encouraging because a high level of precision with GPS is demonstrated even with stations located outside a fiducial network and when wet tropospheric path delays are significant, typically exceeding 20 cm at zenith. ¿ American Geophysical Union 1988