One mosquito opsin gene, NYD-OP7, has been cloned from Culex pipiens pallens. An open reading frame (ORF) of 1116 bp was found to encode a putative 371 amino acids protein which exhibits high identity with opsins from Aedes and Anopheles mosquitoes. Transcript expression of NYD-OP7 was determined by real-time PCR in all life stages of deltamethrin-susceptible and -resistant strains of the Culex mosquito. The results demonstrated that this gene is expressed at all developmental stages, and it is expressed predominantly at the pupae and adult stages. Meanwhile, in pupae and adults, NYD-OP7 is overexpressed in deltamethrin-resistant strain than in -susceptible strain. Importantly, stable expression of NYD-OP7 in the mosquito C6/36 cells can confer moderate deltamethrin resistance. Our study provides the first direct evidence that increased expression of an opsin gene may play some role in the development of deltamethrin resistance in Cx. pipiens pallens. © 2006 Elsevier Inc. All rights reserved.