Tremor disease (TD) of the Chinese mitten crab Eriocheir sinensi has become a serious disease in the Chinese freshwater culture industry in recent years. The agent, belonging to Spiroplasma, was purified from yolk and allantoic fluids of chicken embryos and was then inoculated into the body of crayfish, the abdominal cavity of ICR mice and the allantoid of chicken embryos, respectively. Thirty-eight days after the inoculation, the mice and crayfish were dissected and their tissues sampled and observed with both the light and electron microscope. No infection was detected in mouse or crawfish tissue. But when the different tissues from the inoculated embryonated chickens were tested, the agent was detected only in the brain of embryonated chickens. This indicated that the agent represented a neurotropic characteristic in embryonated chickens, just as it had in the crab. This infective characteristic may be due to the development and maturation of host immunity.