A study was conducted in Suriname on the effects of DDT residual house spray on the main vector of malaria, Anopheles darlingi. Results obtained with an experimental hut sprayed with DDT wettable powder (2 g/m2 AI) were compared with results obtained with an unsprayed control hut. In the sprayed hut, entry rates were reduced by 32% and feeding success by 43.6%. The 24-h mortality of mosquitoes which entered the sprayed hut was 95% (range 85-100%) over the 10-month study period. After furnishing the sprayed hut according to local custom, no reduction in mortality was observed despite an abundance of unsprayed resting surfaces. Bioassays showed that the DDT deposits remained effective for at least 14 months. With excito-repellency test boxes an irritant effect caused by DDT on recently fed An. darlingi females was shown. With a choice-box experiment no preference for unsprayed over sprayed resting places could be demonstrated.