Against a backdrop of vector resistance to insecticides, research efforts are focusing on designing new tools, such as those based on symbiotic bacteria. In this context, Microsporidia MB has been identified as a promising endosymbiont. The aim of this article was to identify the presence of Microsporidia MB in Anopheles gambiae, its seasonal distribution and its distribution according to the vector's life cycle in the Koussinlelego rice-growing area of Benin Republic. Larvae and adult mosquitoes were collected during the dry and rainy seasons at the study site. After collection and morphological identification, PCR was carried out to identify mosquito species and Microsporidia MB. A total of 986 mosquitoes and larvae were collected for this study, of which 4.46% were carriers of Microsporidia MB. In the dry season, 13.72% of the mosquitoes collected carried the symbiont, compared with 0.85% in the rainy season. Also, infection was more prevalent in mosquitoes collected as larvae and reared to adulthood than in those collected as adults in the field. The symbiont Microsporidia MB is present in the Koussinlelego vector population with a higher prevalence in the dry season and among larvae reared to adulthood.