Habitat loss and modification are causing declines in the abundance and distribution of plant and animal species, yet robust information on which to base management and regulatory decisions for these species frequently is not available. Thamnophis gigas (Giant Gartersnakes), a species listed as threatened under the U.S. and California Endangered Species Acts, is strongly associated with aquatic ecosystems in the Great Central Valley (California), yet many aspects of its ecology remain poorly understood. We evaluated relationships between environmental attributes and occupancy of T. gigas and predicted the species' occupancy across ∼300,000 ha in the northern Central Valley. We trapped T. gigas at 159 sites and characterized land cover, land use, and soil type at each site. Occupancy of T. gigas was strongly and negatively associated with elevation and strongly and positively associated with canal density and the proportion of rice and perennial wetland. We also found a strong and previously undescribed association between occupancy and soil order. Estimated occupancy was over five times greater at sites underlain by alfisols, molisols, and vertisols than at sites underlain by entisols and inceptisols. We used the statistical associations between environmental variables and occupancy to predict occupancy at a spatial resolution and extent consistent with management of both T. gigas and regional land and water use.