Two mutually perpendicular, reversed seismic-refraction profiles were obtained by shooting across a point at 40°27′55″N and 73°41′40″W, about 8 miles south of Long Beach, Long Island, and 6 miles east of Ambrose Lightship. Three ground layers were detected with slopes of less than 1° along either profile. The seismic velocities and thicknesses of the layers are: water—4905 ft/sec., 80 feet; unconsolidated sediment—5630 ft/sec., 780 feet; semiconsolidated sediment—6840 ft/sec., 1020 feet; basement—18,600 ft/sec. The precision of the ground layer seismic velocities is estimated as ±5%, that of the layer thicknesses as ±10%. The velocity values and basement-rock depth agree with previous seismic-refraction results for nearby continental shelf areas. The sedimentary layers are tentatively identified with those more exhaustively studied on Long Island itself.