The utilization of natural resources in the Bering and Okhotsk Seas is affected by the presence of highly dynamic seasonal sea-ice covers. The means to observe these ice covers during periods of cloud cover and darkness by the Electronically Scanning Microwave Radiometer (ESMR) which operated on the Nimbus-5 satellite from December 1972 to December 1976, has greatly improved our knowledge of their structure and motion. Both of these ice covers are composed of first-year and younger sea-ice types, and both are highly dynamic because they have unconstrained southern boundaries and exist in areas of intense meterologic activity. Sequential ice-concentration maps derived from ESMR imagery show that both ice covers undergo great variation in ice concentration at short time sca es. Ice concentration and extent for the years 1973 to 1976, as deduced from ESMR imagery, show significant seasonal and annual variations.