Abstract: A neurotoxin, Fraction Ⅸ, was separated from the venom of Bungarus fasciatus by CM-Sephadex C 50 chromatography. The fraction blocked both the neuromuscular transmission of chicken biventer cervicis muscle and the response of the preparation to acetylcholine with a long latent period (134±49 rain). Whereas, the responsibilities of the muscle to direct stimulation or high concentration of potassium chloride were not affected; not many contractures were found in the preparation. In frog sartorius muscle, fraction Ⅸ inhibited the amplitudes of endplate potentials (EPPs) and miniature endplate potentials (mEPPs). Neither the frequencies of miniature endplate potential nor the resting membrane potential of the muscle were changed by fraction Ⅸ. According to the experiments, it was suggested that fraction Ⅸ is a postsynaptic neurotoxin.