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The hydraulic conductance of a large fault zone has been estimated by calibrating a regional groundwater flow model. Drops in groundwater elevations of over 80 m have been observed along a 15- km length of the Mission Creek fault, California, USA. The large drops in elevation are attributed to the reduced hydraulic conductivity of the fault materials. A conceptual and numerical model of the two hydrologic subbasins in Desert Hot Springs, separated by the Mission Creek fault, was developed. The model was used to estimate the hydraulic conductance along the fault. The parameter estimation involved calibrating the model with observed groundwater elevations from over 40 locations over a 60- year period. The fault hydraulic conductances were estimated assuming a linear trend in the fault length, yielding variations in the fault hydraulic conductance of about an order of magnitude along the fault length (2 x 10(-11) - 4 x 10(-10) 1/s). When an average fault thickness of 35 m is assumed, the fault hydraulic conductivity values are estimated to be from three to five orders of magnitude lower than the surrounding materials. A sensitivity analysis indicated that assumptions made in the conceptual model do not significantly affect estimated fault hydraulic conductances.