Methods have been investigated to ascertain the most desirable procedure for determining the trace concentrations of cobalt in sea water. The nitroso-R-salt method of Sandell was adopted and modified for sea water analysis. Samples of water were secured with the nonmetallic water sampler of Thompson and Chow. Preliminary studies of the waters of the San Juan Archipelago and of Puget Sound showed cobalt concentrations ranging from 0.0039 to 0.0054 µg-at Co/I.