Odorant-binding proteins (OBPs) are important molecular players in insect olfaction, which has a great influence on the host-seeking behaviour of mosquitoes and other disease vectors. The mRNA level of the Anopheles gambiae Obp7 gene (Agam-Obp7) is higher in the adult female antennae and is slightly reduced in the female heads after blood-feeding. Here we report the cloning, sequencing, chromosomal mapping and transcript analysis of Aste-Obp7, the Obp7 gene from the Asian malaria mosquito Anopheles stephensi. Quantitative reverse transcription PCR showed that in adult female mosquitoes, Aste-Obp7 was expressed abundantly in the antennae, much less in pooled maxillary palp and proboscis and at the lowest level in the legs. The Aste-Obp7 level in female antennae was significantly higher than in male antennae and it slightly increased 24 h after a bloodmeal. The same pattern held for leg samples as well. The Aste-Obp7 mRNA level dropped more than 10-fold in the female maxillary palp and proboscis after a bloodmeal, although it was still significantly higher than in the males. Together, the above expression profiles suggest that Aste-Obp7 probably functions in female olfaction and may possibly be involved in behaviour related to blood-feeding. We also characterized the Obp7 gene from Anopheles quadriannulatus. Comparison among Anopheles Obp7 genes revealed conserved noncoding sequences that contain potential regulatory elements. The coding sequence and gene structure of Obp7 as well as local synteny of surrounding genes are conserved among the three Anopheles species and two divergent mosquitoes, Aedes aegypti and Culex pipiens quinquefasciatus. OBP7 protein phylogeny is congruent with the mosquito phylogeny and there is evidence of purifying selection acting on the mosquito Obp7 gene. Comparative genomics analysis will improve our understanding of the evolution and regulation of genes involved in mosquito olfaction.