Shallow seismic-reflection surveys conducted near Borah Peak, Idaho, define subsurface structure across a segment of the Lost River fault that was reactivated in 1983. Four different seismic lines using different sets of recording parameters produced reflections from different depth ranges and at different resolutions. The combined interpretation shows the Lost River fault zone in the upper 50 m to be 180 m wide and to be composed of high-angle faults of various ages. The northeast edge of the Lost River fault is clearly defined at a depth of approximately 43¿2 m. The dip of the fault plane in the upper 43¿2m is approximately 55¿ southwest with an uncertainty of 5¿. The interpretation presented correlates with surface structure as well as with deeper geophysical data.