Mass use of insecticide-treated mosquito nets impregnated with permethrin or its derivatives is likely to result in a decline in vector density and transmission of malaria. The present study shows, on the contrary, higher mosquito aggressiveness and an increase in malaria transmission in a district of Douala where alpha-cypermethrin impregnated mosquito nets were recently distributed.Asurvey of the coverage and utilisation of insecticide-treated nets (ITNs) was conducted in February 2016 in the Youpwé district. Night catches of adult mosquitoes on human volunteers were carried out prior to (January 2016) and after (March 2016) alpha-cypermethrin impregnated mosquito nets distribution, in and outside eight randomly selected houses in the Youpwé district. Mosquitoes were identified according to morphological and molecular criteria. The physiological age of mosquitoes was determined according to the Detinova method. Vectors' infectivity was determined by the ELISA CSP test. Long-lasting insecticidal nets coverage and utilisation rates in the Youpwé district were 95.2%and 90 %, respectively. Overall, 3923 mosquitoes were collected within 32 man-night catches. Anopheles coluzzii (24.8 %, n=973) and Culex quinquefasciatus (75.2 %; n=2950) were the only species aggressive to humans. The biting rate was significantly higher after ITN distribution (P=0.024). The aggressiveness cycles were similar during the two phases of the study with a peak of activity between 22:00 and 00:00. The physiological age and infectivity of An. coluzzii were significantly lower before the distribution of ITNs (P<0.004). Poor effectiveness of alpha-cypermethrin impregnated mosquito nets at a dose of 200 mg/m², added to first rainfalls, are the likely causes of the significant increase in aggressiveness and malaria transmission.