Results of measurements of short-period internal waves (periods less than an hour) in the seasonal thermocline in the Black and Caspian seas and some regions of the Atlantic and Indian oceans are discussed. The line sensors of temperature (i.e., the pieces of wire whose electrical resistance varies in accordance with the average temperature of the layer bounded between the ends of the sensor) were widely used. The lengths of the line sensor used were varied from 1 up to 100 m. Measurements with arrays of such sensors in horizontal plane and along the vertical were fulfilled. Records obtained are much more regular than those in cases of point temperature sensors and easy to analyze. It appears often than short-period internal waves exist as groups (trains) of quasi-harmonic oscillations. The frequency and wave number in the group are nonstable. The lowest mode of the oscillations is predominant. Sometimes the waves are standing ones.