In a malaria endemic area in Orissa state in eastern India baseline (November 1989 to October 1990) malaria incidence ranged from 215 to 328 cases/1,000 population/yr in different groups of villages. In November 1990, nylon bednets treated with deltamethrin at 25 mg/m² were given out in two villages (population 1062), untreated bednets were given out in five villages (population 1,226) and in one village (population 786) nets were not given. Nets were retreated in October 1991 and June 1992 in treated-net villages. The trial continued until October 1992. The treated nets caused significant reduction in indoor resting density, biting (landing), light trap catches, human engorgement rate, and parous rate of malaria vector Anopheles culicifacies Giles as compared with untreated nets or no nets. Untreated nets also caused reductions in biting and indoor density. Treated nets retained insecticidal action well over 6 mo. In the final year, malaria incidence was reduced 8.9% in the no-net village, 34.9% in the villages with untreated nets, and 59.1% in villages with treated nets. The relative risk of malaria and parasite rates declined significantly in villages with treated nets. Pediatric splenomegaly rate did not change in the no-net village, increased significantly in villages with untreated nets, but decreased significantly in those with treated nets. Treated nets also reduced pediatric anemia rates, but Hb concentration increased in all villages. Considering the benefits of treated bednets and development of resistance among vectors to DDT and malathion, bednets treated with deltamethrin could be an effective alternative strategy to control malaria in forested areas in India.