In a joint venture, the LBL and CFE have undertaken a series of de resistivity studies at the Cerro Prieto geothermal field. In the present study dipole-dipole resistivity data have been collected and two-dimensional modeling studies have been performed using these and existing Schlumberger data taken by the CFE. The goals of the present research are: (1) to determine whether the reservoir region or controlling geologic structure can be delineated by careful de resistivity measurements and proper interpretation; (2) to establish permanent electrode stations so that measurements may be repeated at regular intervals to determine whether temporal changes in resistivity occur within the reservoir region; and (3) to perform the first set of replicate measurements. During 1978, two long dipole-dipole resistivity lines were surveyed, each with dipole lengths of 1 and 2 km, N = 1 to N = 5. The northern line D-D′ is 18 km long and extends eastward from the Cucapa Mountains into the Mexicali Valley passing just south of the Cerro Prieto volcano. This line follows CFE Schlumberger resistivity line 1. Line E-E′ is 16 km long and was measured several kilometers to the south of D-D′. This line trends subparallel to D-D′ and crosses the central portion of the present production zone. Data were collected with the LBL 25-kW motor-generator and signal-averaging receivers. The error in these measurements has been calculated to be between 5 and 15% at all stations. Two-dimensional modeling studies on both Schlumberger and dipole-dipole data sets yield a compatible model, consistent with known geological features. Several important observations are made from the models. (1) The reservoir region within the production zone is not clearly delineated with de resistivity measurements. This is an expected result, however, since the background resistivity is very low, between 1 and 2 ohm-m. (2) The Imperial and Cerro Prieto faults are clearly delineated in the resistivity models; each fault is characterized by a change in the bulk-section resistivity suggesting that they act as aquacludes.