In fish, vision may be impaired when eye tissue is in direct contact with environmental conditions that limit aerobic ATP production. We hypothesized that the visual acuity of fishes exposed to hydrogen sulfide (H 2 S)-rich water would be altered owing to changes in cytochrome c oxidase (COX) activity. Using the H 2 S-tolerant mangrove rivulus ( Kryptolebias marmoratus ), we showed that a 10 min exposure to greater than or equal to 200 µM of H 2 S impaired visual acuity and COX activity in the eye. Visual acuity and COX activity were restored in fish allowed to recover in H 2 S-free water for up to 1 h. Since K. marmoratus are found in mangrove pools with H 2 S concentrations exceeding 1000 µM, visual impairment may impact predator avoidance, navigation and foraging behaviour in the wild.