A new instrument for the continuous direct measurement of temperature in the upper layer of the sea (within the first millimetre) is described. The technique makes use of a miniature thermistor in a surface-floating hydrophobic support. A wide range of possible applications in fields such as oceanography and climatology exist, and the method also provides a promising alternative to the conventional ‘bottle and reversing thermometers’ or ‘bucket’ methods currently used to provide in situ data for comparison with remote-sensing measurements.