Apteronotus acidops, new species, is described from the upper Paraná River basin, Brazil. It can be diagnosed from the remaining species of the genus by the following combination of characters: dorsal snout profi le pointed; snout length 46.4 – 63.7 % of head length; oculo-nasal distance 39.2 – 72.1 % of postocular distance; lateral ethmoid with both dorsal and ventral extremities expanded, strongly oblique in orientation; mouth rictus surpassing vertical through anterior eye margin, including its skin folds, in all ages; dorsal head profi le nearly horizontal from a vertical through anus to posterior extremity of occipital bone; absence of mid-dorsal white or clear stripe on head and body anteriorly; chin brown; upper lip clear to a vertical through anterior nostril; transversal unpigmented bar or bars posteriorly on the body absent; flanks brown, darker dorsally and pectoral fin hyaline, with chromatophores over rays. Moreover, a key for the apteronotid species from the upper rio Paraná is provided.

Freshwaters, Morphology, Neotropical Region, Ostariophysi, Pisces, South American electric fish, Taxonomy.