We report a case of myasthenia caused by a Mamushi (snake)[Agkistrodon halys Blomhoffi] bite. Electromyography revealed a marked increase in the amplitude of action potentials (waxing phenomenon) with fast rates of motor nerve stimulation and the response to tensilon was negative. This case indicates that Mamushi venoms contain a neurotoxin preventing the release of acetylcholine quanta from nerve terminals.
