Questions have been raised about the quality of indoor residual spraying for malaria vector control after the decentralization of the national malaria control program in Zimbabwe. Given the critical role this control method plays, we conducted an exercise to determine the amount of insecticide (mg active ingredient/m2 of lambda-cyhalothrin) applied during routine house spraying. Severe insecticide underdosing was detected. Spraying efficiency ranged between 63.4 and 76.1% on walls, and 52.7 and 63.2% on roofs. Differences between 2 districts suggested the problem originates from deficient training and lack of pump calibration. Underdosing can undermine effective residual insecticide activity and the expected reduction in disease transmission.