Series of measurements of the depth of an isothermal layer made with bathythermographs or thermocouples simultaneously at three stations indicate strongly the existence of internal waves in the ocean. The properties of the internal waves found in this way are: (1) The wave length is of the order of a few hundred yards; (2) the shorter periods are of the order of a few minutes up to a few hours; and (3) the speed in all cases observed thus far is less than one knot.