A steamflood recently initiated by Mobil Development and Production U.S. at the Lost Hills #3 oil field in California is notable for its shallow depth and the application of electromagnetic (EM) geophysical techniques to monitor the subsurface steam flow. Steam was injected into three stacked eastward-dipping unconsolidated oil sands at depths from 60 to 220 m; the plume is expected to develop as an ellipsoid aligned with the regional northwest-southeast strike. Because of the shallow depth of the sands and the high viscosity of the heavy oil, it is important to track the steam in the unconsolidated sediments for both economic and safety reasons. Crosshole and surface-to-borehole electromagnetic imaging were applied for reservoir characterization and steamflood monitoring. The crosshole EM data were collected to map the i n t m e l l distribution of the high-resistivity oil sands and to track the injected steam and hot water. Measurements were made in two fiberglass-cased observation wells straddling the steam injector on a northeast-southwest profile. Field data were collected before the steam drive, to map the distribution of the oil sands, and then 6 and 10 months after steam was injected, to monitor the expansion of the steam chest. Resistivity images derived from the collected data clearly delineated the distribution and dipping structure of the target oil sands. Difierence images from data collected before and during steamflooding indicate that the steam chest has developed only in the middle and lower oil sands, and it has preferentially migrated westward in the middle oil sand and eastward in the deeper sand. Surface-to-borehole field data sets at Lost Hills were responsive to the large-scale subsurface structure but insuficiently sensitive to model steam chest development in the middle and lower oil sands. As the steam chest develops further, these data will be of more use for process monitoring. This project was initiated from discussions between Ranga Ranganayki at Mobil Research and Michael Wilt at Lawrence Livemore National Laboratory (LLNL) in 1990. The field operators at Mobil Lost Hills became involved with the onset of field adivities in 1991. Much of the technical work was accomplished through the crosshole EM