Glass knifefish (Eigenmannia) are a group of nocturnal weakly electric fishes found throughout the Amazon basin. We made recordings of the electric fields of two populations of freely behaving Eigenmannia in their natural habitats: a troglobitic population of blind cavefish (Eigenmannia vicentespelaea) and a nearby epigean population (Eigenmannia trilineata). These recordings were made using a grid of electrodes to determine the movements of individual fish in relation to their electrosensory behaviors. Cavefish showed continuous evidence of territoriality whereas epigean fish modulated their social behavior in response to diurnal cues. Surprisingly, we routinely found nearby surface fish but not cavefish with sustained differences in electric field frequencies that were below 10 Hz. Such low-frequency differences in electric field frequencies should both cause detrimental sensory interference and trigger the jamming avoidance response. Both surface fish and cavefish showed significant interactions between their electric field frequencies and relative movements at unexpectedly large distances, of at least two meters, and at high differences in frequencies, often greater than 50 Hz. Results General observationsEigenmannia at both the epigean and cave sites were found in clear water streams with rock and sand substrate 6,7,9,12 . At the epigean site, there was a marked diurnal modulation of behavior. During the day, fish were found alone or in groups along the 10/10

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