Observations are given which indicate that the presence of Sargassum below the sea surface is associated with lines of convergence of surface waters and with wind speeds in excess of about 4 m sec-1. It is suggested that descending currents under the convergence lines carry the plants down. Measurements of compression and of rates of rise of S. natans of various densities were made at Bermuda. Results of these measurements indicate that these plants may be carried down by descending currents in excess of 4.5 to 7.2 cm sec-1, and that the plants can maintain positive buoyancy, for short periods of time at least, under pressures equal to 100 m of sea water. The pressure of the gases in the spherical bladders on S. natans was found to change diurnally.