Royal pleco (Panaque nigrolineatus) is endemic from the Orinoco and Amazon basins. Information related to the visual system of this fish is scant, despite this sense is essential to the interactions with the environment. Specimens were classified into two groups based on their weight and standard length: small (standard length (mean +/- SD) of 5.92 +/- 1.43 cm and average weight (+/- SD) 8.50 +/- 6.8 g) and large (standard length (mean +/- SD) of 15.9 +/- 0.69 cm and average weight (+/- SD) 142.48 +/- 18.71 g). The eye of small individuals had a rounded morphology in contrast with the slightly elongated morphology of large individuals. Seven mu m histological sections were obtained and stained with H&E and Orcein techniques. Relative size of the eye was higher in small individuals (6.46 %) than in larger (4.55 %). Corneal central and peripheral thickness presented significant morphometric differences, in both groups. The sclera is composed of dense connective tissue. Larger individuals had a significant thicker lateral and posterior sclera than small individuals. Iris thickness behaves in a similar way. Pupil has omega morphology. Evident cellular interdigitations were located in crystalline lateral regions. P nigrolineatus retina is composed of seven layers. There are not significant differences between retinal thickness of two groups evaluated of P nigrolineatus. There are 10 to 12 optic nerve papillae in P nigrolineatus retina, with different diameters (24.57-131.90 mu m). Histological and morphometric analysis of P nigrolineatus eye fits with adaptations designed to supply visual requirements of benthic organisms that inhabit turbid, low light, with fast currents and structurally complex ecosystems.