An enclosed 32,000-gallon sea-water aquarium with filtration and supply systems and controlled environmental conditions is described. This facility, designed and constructed at the Sandy Hook Marine Laboratory to study the behavior of marine fishes, has been in continuous operation since January, 1965. The elliptically-shaped, multiwindowed aquarium may be operated as an open, closed, or semiclosed circulation system. An artificial lighting system developed for the aquarium room provides approximate natural light intensities and duplicates periods of daylight, twilight, and moonlight.