Abstract: Background and Objective: Closantel is a commonly used antiparasitic drug in sheep rearing areas. However, blindness due to its overdosage has not been evaluated by ultrasonography in the past. The objective of the study was to carry out ocular ultrasonography in sheep (10 female and 8 male) with a history of accidental closantel overdosage vis a healthy sheep. Materials and Methods: The clinical signs were elevated temperature and blindness manifested by bilateral mydriasis, absence of pupillary light and menace reflexes and difficulty in navigation. Ophthalmoscopy revealed an indistinct border of the optic nerve. The ocular ultrasonographic biometric parameters of the affected sheep were compared with age-matched healthy sheep, selected from the affected flocks. Six animals each (three male and three female) were selected for the age group <1 year and 1-3 years. Results: Ocular ultrasonographic examination of healthy sheep revealed significantly (p<0.05) increased anterior-posterior depth of lens, the lateromedial diameter of the lens, vitreous chamber depth and axial length in older animals (1-3 years) as compared to animals of <1 year age group. A similar trend was observed for the affected sheep. The statistical comparison of parameters between healthy and affected sheep revealed significantly (p<0.05) increased optic nerve sheath diameter in both the age groups of affected sheep as compared to the healthy sheep. The rest of the parameters did not differ significantly (p<0.05) between the healthy and affected sheep. Despite symptomatic treatment, none of the sheep regained eyesight. Conclusion: Closantel toxicity in sheep causes irreversible blindness due to increased optic nerve diameter.