Increasing chloride concentrations are a threat to fresh ground-water sources in East Baton Rouge and West Baton Rouge Parishes, Louisiana. Large withdrawals at Baton Rouge have lowered water levels and altered flow patterns in most of the 10 aquifers that underlie the area. Prior to development, freshwater flowed southward to the Baton Rouge fault, an east-west trending growth fault that extends through Baton Rouge and across southeastern Louisiana. Aquifers south of the fault generally contain saltwater. Ground-water withdrawals north of the fault have created gradients favorable for the movement of saltwater from south of the fault into freshwater areas north of the fault. Water samples were collected from 152 wells during 2004-05 to document chloride concentrations in aquifers underlying East and West Baton Rouge Parishes. The background concentration for chloride in fresh ground water in the Baton Rouge area north of the Baton Rouge fault is generally less than 10 milligrams per liter. Chloride concentrations exceeded 10 milligrams per liter in one or more samples from wells north of the fault screened in the '600-foot', '1,000-foot', '1,200-foot', '1,500-foot', '1,700-foot', '2,000-foot', '2,400-foot', and '2,800-foot' sands. Comparison of the 2004-05 data with historical data indicated that chloride concentrations are increasing at wells in the '600-foot', '1,000-foot', '1,200-foot', '1,500-foot', '2,000-foot', '2,400-foot', and '2,800-foot' sands north of the Baton Rouge fault.