The specific activity of Silicon-32 is measured is near-coastal seawaters using anin situ method for the extraction of dissolved silica. The results are discussed in relation to the input of Si³² at the surfacevia rains, the expected variation in its concentrations due to continental run-off and influx of open ocean surface waters. The experimental results clearly demonstrate the usefulness of Si³² as a tracer for the study of the vertical structure of the ocean in the upper layers, and of seasonal variations in the types of waters entering in the coastal regions.