To investigate the effectiveness of the malaria control program, the behavior of Anopheles darlingi females was studied following spraying of DDT on the walls of house-holds along the Ituxi river in the state of Amazonas, Brazil. The study was carried out on two four-walled dwellings, one of which was sprayed with 2 g of DDT per m2 of wall. Three methods were used to study the mosquitoes' activity before and immediately after the spraying, as well as at 2 and 12 months post-spraying. These methods were: capturing the mosquitoes in the act of resting on human baits, capturing them in traps as they entered or exited the dwellings, and liberating females tagged with fluorescent powder inside the house and then following them with ultraviolet light. Immediately after the spraying, the females stopped going in and out of the house and ceased biting inside the sprayed dwelling. In addition, the tagged females that had been set loose inside the house fled almost immediately. These phenomena were not observed in the unsprayed dwelling. The reaction of the A. darlingi to spraying is considered to indicate true repellency and not simply irritation from contact. Since the dwellings in the locality have only 2.2 walls on average, the persistence of malaria in the territory could be due to the type of household construction.