The nature of pyrethroid resistance in Aedes aegypti Brazilian populations was investigated. Quantification of enzymes related to metabolic resistance in two distinct populations, located in the Northeast and Southeast regions, revealed increases in Glutathione- S- transferase (GST) and Esterase levels. Additionally, polymorphism was found in the IIS6 region of Ae. aegypti voltage-gated sodium channel ( AaNa V ), the pyrethroid target site. Sequences were classified in two haplotype groups, A and B, according to the size of the intron in that region. Rockefeller, a susceptible control lineage, contains only B sequences. In field populations, some A sequences present a substitution in the 1011 site (Ile/Met). When resistant and susceptible individuals were compared, the frequency of both A (with the Met mutation) and B sequences were slightly increased in resistant specimens. The involvement of the AaNa V polymorphism in pyrethroid resistance and the metabolic mechanisms that lead to potential cross-resistance between organophosphate and pyrethroids are discussed.