The southern house mosquito, Culex quinquefasciatus Say, is a competent vector of human disease and an important target of mosquito abatement programs. However, these management programs have been compromised by development of insecticide resistance. In the current study, susceptibilities to naled and resmethrin, two adulticides used in mosquito abatement, were monitored using a topical and contact bioassay, respectively, in five field- collected populations of C. quinquefasciatus (MARC, HOOD1, HOOD2, MINLOVE, and THIB). Frequencies of resistance, measured as survival after treatment with discriminating concentrations (i.e., sufficient to kill>90% of a reference susceptible strain) were high (88.0–96.8%) in all field collections treated with naled, but were variable (3.3–94.2%) with resmethrin. In addition, esterase activities in mosquitoes from these collections were quantified using α-naphthyl acetate and ranged from 1.08 to 3.39 µmol α-naphthol produced min^-1 mg prot^-1. Heightened activities were associated with decreased insecticide susceptibility in HOOD1, THB3, and MINLOVE but not HOOD2. Esterases were visualized using native Polyacrylamide gel electrophoresis, and intra- and interstrain differences in banding patterns were detected. In addition, esterases from MINLOVE mosquitoes were more numerous and intensely staining when compared with those from a laboratory-susceptible strain. Finally, naled synergized the toxicity of resmethrin in populations with decreased insecticide susceptibility and increased esterase activity by 2.5– (MINLOVE) to three-fold (THIB). Results from this study will allow management strategies for populations of C. quinquefasciatus to be optimized, and provide a foundation for further studies exploring use of esterase inhibitors as synergists of pyrethroid toxicity.