The southern house mosquito, Culex quinquefasciatus Say (Diptera: Culicidae), transmits pathogens and is one of the most important vectors of human disease. The carboxylesterase mosquito genes encoded esterase enzyme involved in the mechanism that provides resistance to insecticides. We amplified by RTPCR four alternative transcripts from the esterase gene not reported previously (A, B, C, and D) from Cx. quinquefasciatus larvae from Monterrey, N.L., northeastern Mexico. Through cDNA sequences, we predicted amino acid sequences used to model the three-dimensional structure of protein isoforms. We determined d_N and d_S ratios to identify evolutionary forces which lead the family. PCR products were obtained from 5A esterases (1839, 1898, 1894, and 1953 bp). The translation sequence of the PCR products showed polypeptides of 612, 473, 282, and 282 amino acids in length for isoforms A, B, C, and D, respectively. Three exons and two introns were obtained in the analyzed sequences. The study sequences were compared with those previously published showing 98.7, 98.5, 91.5, and 91.5% amino acid similarity for isoforms A, B, C, and D, respectively. The threedimensional models showed that only ORF-D had the three amino acids corresponding to the catalytic triad, while ORF-A and ORF-C lacked one or two of the three amino acids. Esterase genes are under purifying of selection; a clue the members are functional. The processes might be useful in monitoring evolutionary dynamics of known carboxylesterase genes and identifying new carboxylesterase splicing.