The Santa Cruz valley in Arizona is a rapidly urbanizing area. Complaints by residents of the area about pest mosquitoes prompted the investigation of mosquito breeding sources and a search for management techniques that would reduce mosquito populations. Many types of mosquito breeding sites were found in the area, and eight species of mosquitoes were identified. The greatest source of mosquitoes was a 2400 hectare irrigated pecan orchard. The most numerous and annoying mosquitoes were Aedes vexans (Meigen) and Psorophora columbiae (Dyar and Knab). The orchard is irrigated ca. every two weeks from April through October by flooding level areas, called borders, between rows of trees. Mosquitoes hatched with every irrigation studied, from April through September, but reached annoying numbers from late April through mid-September. A. vexans was the dominant species in early spring and P. columbiae was dominant in summer. Highest populations were reached coincident with the summer rainy season in July and August. Tests were performed to determine the efficacy of Bacillus thuringiensis israelensis (B.t.i.) as an additive to irrigation water for control of mosquito larvae. The larvicidal material was effective at all concentrations from .586 to 2.344 l/ha and with all techniques used. The most cost-effective application technique was to use fertilizer tanks to drip a mixture of B.t.i. into irrigation water in ditches before the water ran into fields. The general rate of three parts larvicide per million parts irrigation water was found to be effective, especially when supplemented with a spray of one part larvicide to 64 parts water applied to the ends of borders two to three days following irrigation. An increase in larvicide concentration was found to be necessary during the peak of mosquito season. A successful management program was developed and applied for 1 year, during which no mosquito complaints were made by citizens, the population of mosquitoes in the orchard was reduced to a point where farm laborers were no longer annoyed, and farm managers were satisfied that the program was cost effective.