Thorium concentrations were determined in filtered (0.45‐µ Millipore) and unfiltered seawater as well as in marine algae. In filtered seawater, values were 8 × 10−13 g Th liter−1 and lower; for unfiltered seawater samples, values about three orders of magnitude higher were obtained. Thorium concentrations in marine algae are in the range of 10−6 to 10−8 g Th g−1 ashed material.