The mechanism underlying the ocular signs associated with severe cases of bothrop bite is unknown. We subjected one case to various examinations including MRI of the brain, electrooculography and the tensilon test. The patient, a healthy 20-year-old male, had been bitten in the right index finger. Eight hours after the injury, he showed swelling over the right shoulder and ocular signs including double vision and ptosis. He was diagnosed as having a severe grade of bothrop bite associated with oculomotor and abducens palsies. No other neurological signs, including synergestic divergence, were detected and the brain MRI revealed no abnormal intensities in the midbrain. The tensilon test was negative. The electroculogram showed paralysis in the bilateral inferior oblique muscles, the left inferior rectus muscle, the left median rectus muscle and the left lateral rectus muscle. Administration of antiserum improved the swelling and the ocular signs. On the 39th hospital day, he was discharged with no neurological deficits. It is assumed that the mechanism of ocular signs in cases of bothrop bite is neuromuscular blockade by a neurotoxin.