A reconnaissance geophysical survey of the Barents and Kara Seas, conducted from icebreakers in 1965 and 1966, consisted of about 10,000 line-km of bathymetric-magnetic data, 57 gravity measurements, and two short end-to-end seismic refraction profiles. Magnetic data do not support a continuation of Caledonian or Precambrian basement structures under the Barents Sea at shallow depth. A broad 400-gamma anomaly, the source of which lies about 20 km below sea level, strikes north-northwest between Norway and Bear Island. Magnetized basement approaches the sea floor only near Bear Island and 74.5°N, 33°E. Free-air anomalies in the northeast Barents Sea are within 30 mgal of zero. Magnetic anomalies in the Kara Sea parallel, but do not connect, tectonic structures of T ymyr and Novaya Zemlya. Magnetic basement is deep in the southwestern Kara Sea but shoals northeast of a line connecting northern Novaya Zemlya with the Yeniseyskiy Zaliv (estuary). Thus, primarily vertical movement is suggested along the line. Anomalies with amplitudes greater than 500 gammas occur only near the pre-Mesozoic outcrops on the Izvestiy Ts.I.K. Islands. The East Novaya Zemlya Trench is slightly undercompensated, having a median free-air gravity of -9 mgal and extremes of -33 and +11. Two refraction measurements in the eastern Barents Sea showed 400 m of unconsolidated sediments overlying a 3.1-km/sec basement and 700-1,100 m of 2.8.km/sec sediment overlying a 4.1-km/sec basement. The basement is probably Paleozoic sandstone or shale.