Abstract: Plankton is distributed in all oceans and seas, and therefore its acoustic properties can be essential for underwater acoustics. A sound scattering cross section of plankton can increase due to gas cavities associated with plankton cells. Such gas cavities has been found in many kinds of phyto- and zoo- plankton. The present paper gives results of investigation of plankton gas cavities with different methods. II was found in particular that sound velocity in phytoplankton suspensions varies both up and down compared to the sound velocity in pure water. A response of phyto- and zoo- plankton on compression and decompression is studied. It was found that species of euphausiids having long range vertical daily migration use bubbles (swimbladders) lo control their buoyancy, while species living without long vertical migration do not have swimbladders.