In prokaryotes and eukaryotes, the Period (PER) gene is important for the circadian clock that regulates some rhythmic biochemical, physiological, and behavioral processes. PER messenger amplification was done by RT-PCR from southern house mosquito, Culex quinquefasciatus Say, larvae collected at three locations in the State of Nuevo Leon, northeastern Mexico. The amplified products were cloned, sequenced, and compared/analyzed with those of other species of mosquitoes. A 3,093-bp amplified product was synthesized by PCR, which is the complete coding DNA sequence (3,027 bp) of PER encoding an open reading frame of 996 amino acid residues and much smaller than the orthologous in other mosquitoes analyzed. No other simultaneous amplification was visualized; thus, possible alternative splicings were discarded or at least not detected by the method. Because PER is a widely conserved gene in prokaryotes and eukaryotes and maintaining colonies of southern house mosquitoes is relativity easy, determining the sequence of the gene in the mosquito provides the possibility of using the gene as a model and generating data on the role in the circadian clock.