Insecticide resistance in the main malaria vectors in Africa is a major concern for malaria vector control program managers. The most common insecticides used for indoor residual spraying (IRS) and treating bed nets are becoming increasingly ineffective. The quest for safer and more effective insecticides for malaria vector control is urgent. This study sought to evaluate the efficacy of ACTELLIC 50 EC (pirimiphos methyl), an organophosphate, for IRS in Ghana, where there is high vector resistance to pyrethroids and organochlorines. Before the commencement of the study, standard World Health Organization (WHO) vector susceptibility tests against a common malaria vector, Anopheles gambiae s.l, were conducted using preparations of pyrethroids, organochlorines, carbamates, and organophosphates. The vector was found to be resistant to the pyrethroids, the organochlorines, and the carbamates, but susceptible to the organophosphates. The emulsifiable organophosphate concentrate formulation, ACTELLIC 50 EC, was then evaluated to determine the efficacy and the length of its residual effect. The wall bioassay test, using recommended cones from WHO, was conducted on sprayed surfaces with ACTELLIC 50 EC from 27 July 2009 to 16 October 2009. After 15 wk of trials on painted cement surface, it was found out that the main malaria vector, An. gambiae s.l, was susceptible to the insecticide even though the WHO Pesticide Evaluation recommends 2- to 3-mo duration of effective action. Therefore, it is recommended for use in IRS programs in this part of Ghana, where there is high vector resistance to most of the insecticides.