Some predominantly marine fish families include a small proportion of species that are obligate freshwater inhabitants, and there is accumulating evidence for obligate freshwater species within the family Muraenidae. Whether the muraenid Gymnothorax polyuranodon (Bleeker, 1854) can survive, grow and coexist over an extended period in captive freshwater conditions was tested following repeated observations of this species in freshwater streams of the Australian Wet Tropics. Changes in the body size of four individuals held in a 1200 litre freshwater aquarium revealed that yearly growth ranged from a minimum of 21.0 cm total length (TL) and 2.4 times body mass to 26.5 cm TL and 3.9 times body mass. Maximum daily ration of individuals (fed worms, prawns and fish) ranged from 3.4% to 3.9% of body mass. Individuals coexisted peacefully, with only brief bouts of intraspecific aggression that included biting. While the full life cycle of G. polyuranodon remains unresolved, the current study reinforces field observations and microchemistry insights indicating that this species can occupy and grow in freshwater ecosystems, and points toward the likelihood that the species probably plays an important and previously unrecognised mesopredatory role.