Dispersive optical hysteresis in an all optical passive unidirectional ring cavity containing a molecular gas has been observed. Ammonia is used as the nonlinear medium off resonantly excited on the aR(1,1) transition using temporally smooth 100-ns pulses from a transversely, excited, atmospheric pressure CO2 laser. Results are in excellent agreement with the theory of Ikeda recently generalized by the authors to describe period doubling in this system.