Optical bistability in monocryst. Pb(Mg1/3Nb2/3)O3, (PMN) was studied at temps. near the ferroelec. phase transition under 514.5 nm illumination. At moderate laser power densities above a threshold but <100kW/cm2, this material exhibited reproducible aperiodic oscillations following the rapid switching on of a const.-intensity Gaussian laser beam. With increasing incident power the oscillations changed from the sym. type obsd. in PLZT to a sequence of rapid jumps followed by a slow relaxation between two metastable thermal lens configurations, similar to those obsd. in ceramic samples of PMN at lower power densities (~1 kW/cm2). The duration and no. of the oscillations depended reproducibly on incident power and ambient temp., with different spatial structures from those obsd. in ceramic samples, depending not only on incident beam power but also on incident beam width and wavefront curvature.