Gymnotus coatesi, a small and rare species of Neotropical electric fish, was originally described by LaMonte from material collected in Pará, Brazil, in 1934. The senior author discovered populations of this species in the vicinity of Tefé, Amazonas, Brazil, and near Iquitos and Jenaro Herrera in the lowland Upper Amazon of Peru. We provide a formal redescription of this species and describe external morphology, osteology, ecology, and electric signals, with emphasis on the Tefé population. Gymnotus coatesi can be distinguished from all congeners by a unique banding pattern and by a unique combination of morphometric and meristic characters. Gymnotus coatesi is known from localities along the main stem of the Amazon River from near the confluence of the Río Marañon with the Río Ucayali (Loreto, Peru) to the Rio Tocantins (Pará, Brazil) but is poorly represented in museum collections. In the Tefé region, G. coatesi inhabits the seasonally flooded lower courses of terra firme rain-forest streams. In the Peruvian Amazon, it also occurs in the upper, nonflooded, reaches of rainforest streams.

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