We report a 29-year-old woman who developed unilateral unreactive mydriasis and cycloplegia after 5 days of persistent constipation. During the next hours the patient complained of dry mouth and difficulties in swallowing food; iris and ciliary muscle palsies spread over the second eye. Ocular motility was normal and there were no clinical signs of neuromuscular involvement. Conventional electromyography and evoked muscle action potentials following repetitive nerve stimulation were normal; single-fiber electromyography showed normal jitter and absence of blocking. The diagnosis of botulism was considered as most likely, and the patient was given botulinum antitoxin. The post-treatment course was characterized by bilateral tonic pupillary reaction to near, sectoral iris contractions to light and pupillary constriction to 2 mm in 40 min following topical instillation of 0.1% pilocarpine. Ocular manifestations completely disappeared within 5 weeks. Botulism type B toxin was demonstrated in the pretreatment stool of the patient but not the serum.