  A decay of Type III burst into two Type III bursts was registered during
solar observations by GURT and URAN-2 radio telescopes on April 18, 2017. Such
phenomenon was observed for the first time. Newborn Type III bursts have drift
rates smaller than that of decaying Type III burst. Such decays of Type III
bursts were predicted by a gas-dynamic theory of high-energy electron beams
propagating through the thermal background plasma. In the frame of this theory
Type III sources are beam-plasma structures moving with constant velocities. In
our case the sum of velocities of newborn Type III sources equals the velocity
of decaying Type III source. The last one is 0.33c in the case of fundamental
radio emission and 0.2c at the harmonic radio emission of Type III burst. The
density ratio of slow and fast newborn Type III sources is about 3.
