1. Ampullary organs of the fresh water stingrayPotamotrygon have short canals. Receptor cells of the ampullary epithelium have a single kinocilium on their lumenal faces and form ribbon synapses with afferent fibers on their basal faces. 2. Afferents from the receptors exhibit a tonic rhythmic discharge. Outside negative stimuli in the millivolt range accelerate the discharge. Outside positive stimuli decelerate the discharge. 3. Polarity of response and presence of a kinocilium indicate that these receptors are homologous with ampullae of Lorenzini. Shortness of the canals and low sensitivity can be explained as adaptations to the high impedance of the fresh water environment.