A 21-year-old Thai man first presented with eosinophilic meningitis. One week later he developed visual impairment of the left eye with a visual acuity of 20/600. He had a Marcus Gunn pupil, constricted visual field, prolongation of visual evoked potential and the presence of inflammatory cells in the vitreous cavity of the affected eye. On funduscopic examination there was disc swelling with hyperemia of the left eye. These ocular findings are compatible with optic neuritis. The causative agent, Angiostrongylus cantonensis, was identified in the vitreous cavity. To our knowledge this is the first case of optic neuritis caused by intraocular angiostrongyliasis. Ten days after administration of corticosteroid his severe headache was resolved, and by 4 weeks the disk swelling of the left eye subsided. Eight months after treatment the visual acuity of the left eye had not improved due to permanent damage to the retinal pigment epithelium caused by the intraocular parasite.