The synthetic pyrethroid deltamethrin was evaluated as a 15 mg/m2 residual application of a microencapsulated formulation for efficacy in malaria control in a 50 km2 area of north-eastern Zimbabwe. Results were compared with very large contiguous DDT sprayed (2g/m2) and unsprayed areas. A total of 3544 rooms were sprayed with deltamethrin. No significant side effects of the insecticide on spraymen were noted. Mosquito captures were poor and inconclusive due to drought conditions, but the malaria vectors Anopheles arabiensis and An. gambiae were identified in the area, the former being 14 times more abundant than the latter in bait collections from an unsprayed area. Monthly bioassays using laboratory-reared An. arabiensis showed a higher mortality of mosquitoes exposed on roofs for up to eight months after deltamethrin application and indicated a high mortality in deltamethrin sprayed houses than in DDT sprayed houses. Tests indicated that, after eight months, up to 50% of the deltamethrin remained on the roof and 25% on the wall. Malaria transmission, evaluated by blood slide surveys at the beginning and end of the transmission season, was high in the unsprayed area, slightly less in the adjacent and incompletely isolated deltamethrin sprayed area, and absent or very low in the DDT sprayed area. The results were considered to be favourable for the use of deltamethrin in malaria control by means of residual house-spraying with the non-irritant dosage of 15 mg/m2, but require evaluation in larger trials.