Of the 20 Anopheles species caught in villages in Sonapur, Assam, only An. minimus was incriminated as a malaria vector by finding sporozoites in the salivary glands. It was found to be endophagic and endophilic in Assam and because its biting peaked after midnight it was a suitable target for insecticide impregnated bednets. After the withdrawal of DDT spraying and collecting a year's baseline data, deltamethrin impregnated nets were distributed in 3 villages, untreated nets were distributed in 6 villages and 3 were held as untreated controls. The population of each of these groups of villages was about 1700. The nets were well received by the local tribal population. Human landing catches with baits unprotected or under partially lifted nets showed that the nets provided a high degree of personal protection against all the local species of human biting mosquito. In addition, there was evidence for suppression of the An. minimus population in a village with treated nets. Malaria was monitored by weekly active surveillance in all the villages. In the untreated control villages the slide positivity rate and monthly parasite index rose significantly during the trial. In the villages with untreated nets, these parameters showed no significant change, but in the villages with treated nets they declined significantly. On the basis of these results, the widespread distribution of impregnated nets was recommended to the state health authorities.