We previously showed that gene expression in the midgut of female Culex nigripalpus Theobald (Diptera: Culicidae) was altered after ingestion of a bloodmeal and some of the expressed cDNAs showed stage-, sex-, and tissue-specific expression. One of these expressed cDNAs, CN G11B, is here shown to be similar to members of a gene family that encodes odorant-binding proteins. CN G11B.1 is a cDNA fragment of 721 bp, is incomplete at the 5'-end, and contains a canonical polyadenylic acid tail signal sequence in the 103-bp 3'-untranslated region. Translation of CN G11B.1 provides a putative protein product of 207 amino acids. GenBank tblastx, VectorBase, and Pfam database searches showed identity to hypothetical proteins from Culex pipiens quinquefasciatus Say (86%) and Aedes aegypti (L.) (55%). These proteins have structural motifs similar to those found in the gene family that includes odorant-binding proteins. CN G11B.1 putative protein has two of the six cysteines that are highly conserved in most invertebrate odorant-binding proteins. CN G11B.1 expression increased in thoraces (6-12 h) and in abdomens (3-48 h) of blood fed female Cx. nigripalpus but was not detected in RNA from heads, indicating a possible role in both chemoreception and blood feeding.