Optical bistability in ferroelectric ceramic lead magnesium niobate at room temperature was discovered recently (Chenet al., Applied Physics Letters,60, 332, 1992) and characterized in detail as a function of laser power, sample temperature, pressure and flow rate of the ambient air (Scott and Chen,Integrated Ferroelectrics,1, March 1992). In all these experiments the laser light was incident on the PMN sample with a very fast rise time; thus the switched thermal response of the system was isothermal, with relatively rapid relaxation to steady-state temperatures that are functions of input power. By comparison, in the present study the laser light intensity is ramped up slowly and linearly (slowly compared to the thermal relaxation time of the system, which is typically 200 ms). Under these circumstances the thermal focussing responds adiabatically, and different spatial and temporal patterns are observed. Notable is the hysteretic behavior associated with each switching event; the onset of switching differs for increasing and decreasing laser power by approximately 10–30 mW (10–30 KW/m2), for input powers of 0.1–0.4 W.