Description: Background: The doublesex gene controls somatic sexual differentiation of many metazoan species, including the malaria mosquito Anopheles gambiae and the dengue and yellow fever vector Aedes aegypti (Diptera: Culicidae). As in other studied dipteran dsx homologs, the gene maintains functionality via evolutionarily conserved protein domains and sex-specific alternative splicing. The upstream factors that regulate splicing of dsx and the manner in which they do so however remain variable even among closely related organisms. As the induction of sex ratio biases is a central mode of action in many emerging molecular insecticides, it is imperative to elucidate as much of the sex determination pathway as possible in the mosquito disease vectors. Results: Here we report the full-length gene sequence of the doublesex gene in Culex quinquefasciatus (Cxqdsx) and its male and female-specific isoforms. Cxqdsx maintains characteristics possibly derived in the Culicinae and present in the Aedes aegypti dsx gene (Aeadsx) such as gain of exon 3b and the presence of Rbp1 cis-regulatory binding sites, and also retains presumably ancestral attributes present in Anopheles gambiae such as maintenance of a singular female-specific exon 5. Unlike in Aedes aegypti, we find no evidence for intron gain in the female transcript(s), yet recover a second female isoform generated via selection of an alternate splice donor. Utilizing next-gen sequence (NGS) data, we complete the Aeadsx gene model and identify a putative core promoter region in both Aeadsx and Cxqdsx. Also utilizing NGS data, we construct a full-length gene sequence for the dsx homolog of the northern house mosquito Culex pipiens form pipiens (Cxpipdsx). Analysis of peptide evolutionary rates between Cxqdsx and Cxpipdsx (both members of the Culex pipiens complex) shows the male-specific portion of the transcript to have evolved rapidly with respect to female-specific and common regions. Conclusions: As in other studied insects, doublesex maintains sex-specific splicing and conserved doublesex/mab-3 domains in the mosquitoes Culex quinquefasciatus and Cx. pipiens. The cis-regulated splicing of Cxqdsx does not appear to follow either currently described mosquito model (for An. gambiae and Ae. aegypti); each of the three mosquito genera exhibit evidence of unique cis-regulatory mechanisms. The male-specific dsx terminus exhibits rapid peptide evolutionary rates, even among closely related sibling species.