The gill morphologies of six species of potamotrygonid freshwater stingrays from the Amazon basin were investigated using light and electron microscopy. Some unique features were found in the potamotrygonid gill: (1) fingerlike protuberances on the gill filament, (2) an Alcian blue/periodic acid-Schiff-positive histochemical reaction for several cell layers in the gill epithelium (except the basal ones), (3) pavement cells with numerous subapical mucous vesicles, (4) very large mucous cells, and (5) follicular Na(+)/K(+)-ATPase-rich (NKA-rich) mitochondria-rich cells (MRCs) in Potamotrygon sp. (known as the cururu ray). The fingerlike protuberances may constitute an additional resistance to water flow, helping to drive water through the lamellae. The secretion of a mucous substance by the pavement cells and mucous cells may help to protect the gills against mechanical injury and pathogens and aid in osmoregulation in the dilute water of the Amazon basin. All MRCs possess enfolded basolateral membranes and have poorly developed or absent tubular systems. NKA-rich MRCs are located high in the basolateral membrane. The cururu ray, which is endemic to the Rio Negro, has follicular NKA-rich MRCs (8-12 cells in cross section) that share the same apical pit in the filament; this may be considered to be an autapomorphy. The combination of these branchial characteristics may have favored tolerance to the freshwater environment during the evolution and diversification of potamotrygonids throughout the Amazon basin.