A ground magnetic study was completed in the Coso volcanic field to investigate faulting and associated hydrothermal alteration patterns. The magnetic intensity contours match general geologic patterns in varying rock types. Hydrothermally altered rocks along intersecting fault zones show up as strong magnetic lows that form a triangular-shaped area. This area is centered in an area of highest heat flow and is a site of concentrated fumarolic activity. In the Coso volcanic field the combination of high heat flow, fumarolic activity, magnetic lows, and hydrothermal alteration along faults suggests that hot fluid filled fractures with high permeability.