Abstract: An animal enclosure system comprising an enclosure and a modular assembly of enclosures. Each enclosure is adapted so that environmental conditions may be easily and economically changed and conformed to the particular animal in that enclosure. A selection of different levels for the animal and access means for the animal between levels is provided. An adjustable drain and overflow assembly is provided, adapted to permit easy, selective, accurate and reliable adjustment of the water level in the enclosure and to prevent inundation, even if there is a continuous influx of water. Wet-dry environment balance is carefully controllable.