Malaria transmission has been studied in Djoumouna, south of Brazzaville between October 2016 and May 2017. The objective of the study was to evaluate the transmission of malaria. The captures of the residual fauna were made according to three situations, the absence, the presence or the defectiveness of the mosquito nets. The mosquito nets observed were originally long-lasting insecticide-treated mosquito net (LLIN) acquired during the 2011-2012 mass distribution campaign. Efficacy tests conducted with these nets reveal very low mortality rates (ranging from 2.5% to 17.5%). This mortality indicates the almost non-existence of the insecticidal effect, therefore, these LLIN were considered as simple nets. Anopheles gambiaes.l. at 99.68% was predominant, followed by Culexquinquefasciatus at 0.32%. The highest anopheline density was observed in rooms without mosquito nets, followed by rooms with defective nets. On average, malaria transmission is infective per person every two weeks in rooms without mosquito nets; it is an average of one infective bite per month, in the rooms with defective mosquito net and infective sting every 6 months in rooms with mosquito net in good condition. The use of healthy LLIN in 2012 has been an important factor in reducing malaria transmission. In 2017, the deterioration of these LLIN or their absence has been an element in the rise of malaria transmission.