AbstractInsecticide resistance is currently a threat to the control of Aedes agypti, the main vector of arboviruses in urban centers. Mutations in the voltage gated sodium channel (Na_V), known as kdr (knockdown resistance), constitute an important selection mechanism for resistance against pyrethroids. In the present study, we investigated the kdr distribution for the Val1016Ile and Phe1534Cys alterations in Ae. aegypti from 123 Brazilian municipalities, based on SNP genotyping assays in over 5,500 mosquitoes. The alleles Na_VS (1016Val⁺ + 1534Phe⁺), Na_VR1 (1016Val⁺ + 1534Cys^kdr) and Na_VR2 (1016Ile^kdr + 1534Cys^kdr) were consistently observed, whereas kdr alleles have rapidly spread and increased in frequency. Na_VS was the less frequent allele, mostly found in Northeastern populations. The highest allelic frequencies were observed for Na_VR1, especially in the North, which was fixed in one Amazonian population. The double kdr Na_VR2 was more prevalent in the Central-west and South-eastern populations. We introduce the ‘kdr index’, which revealed significant spatial patterns highlighting two to three distinct Brazilian regions. The 410L kdr mutation was additionally evaluated in 25 localities, evidencing that it generally occurs in the Na_VR2 allele. This nationwide screening of a genetic mechanism for insecticide resistance is an important indication on how pyrethroid resistance in Ae. aegypti is evolving in Brazil.