Enzymatic mechanisms indicating resistance were determined in the southern house mosquito, Culex quinquefasciatus Say, from Nuevo Leon, northeastern Mexico, after exposure to pyrethroids: trans-permethrin (92% trans-6% cis) 0.75%, bifenthrin (99%) 0.1%, cypermethrin (99%) 0.1%, deltamethrin (99%) 0.5%, and lambda-cyhalothrin (99.1%) 0.5%. Mosquitoes were collected from four municipalities in the north and center of the state of Nuevo Leon. Four groups of 25 female mosquitoes, 24 hours of age were exposed to insecticide doses pre-evaluated. After 50% of the mosquitoes had died, individuals were divided into alive and dead categories. With the individuals selected and a group of 100, 1-day old adult females from each location without exposure to the pyrethroids, biochemical tests were used to determine resistance-related activities of enzymes alpha- and beta-esterases, glutathione S-transferases, and mixed-function oxidases. The results were compared with those from a susceptible strain of southern house mosquito obtained after rearing for 25 generations in a laboratory. Biochemical assays showed elevated glutathione S-transferases and esterases, enzymes associated with resistance to pyrethroids.