The Tres Rios Constructed Wetlands Demonstration Project was planned, designed, and constructed to provide habitat, water treatment, low-impact recreation, and research opportunities in Maricopa County, Arizona. The wetlands area is split into three sites, with approximately 13 acres of emergent marsh, free-water surface wetlands. Research has concentrated on water quality improvements, vegetation sustainability, mosquito control, and wildlife attraction. Soon after completion of the wetlands, beavers moved in. The arrival of the beaver was a sign that the wetlands were doing their job in attracting native wildlife and providing habitat that has been severely degraded in the last 100 years. Then, the beavers multiplied. Using a nighttime spotlight survey, beaver numbers were estimated at somewhere between 34 and 50 individuals. Other evidence of excessive beaver activity included extensive damage to wetland plants from foraging, cut or girdled trees, burrows, and runways. After consultation with Wildlife Services, a cooperative non-lethal beaver research program was established. This paper describes the two phases of the program. The research currently being conducted by Wildlife Services and Tres Rios staff is vital in maintaining acceptable populations of beavers within the full-scale Tres Rios project. The research is currently at a very early stage, but will continue for approximately four more years, providing the opportunity to learn much about beaver ecology in the southwestern United States, and assess efficacy of non-lethal control options. Includes 4 references.