We present the first experimental observation of phase conjugate (PC) secon
d sound (SS) waves superfluid helium. The main feature of a PC wave is that
its phase is complex conjugate to the Incident wave phase. It is generated
and amplified as a result of nonlinear interaction between an incident SS
wave and a first sound (FS) pumping wave. At FS amplitudes larger than the
threshold value, a parametric instability, i.e., spontaneous decay of a FS
wave into two SS waves, takes place. Three main, theoretically predicted, f
eatures of phase conjugate waves were verified experimentally.