Insect odorant-binding proteins (OBPs) are small, water-soluble molecules that are thought to transport the hydrophobic odorants to their receptors in the chemosensory neurones. Here we report the identification and molecular characterization of the Anopheles stephensi odorant-binding protein 1 gene (AsteObp1), an Obp1 gene in An. stephensi, a major malaria vector in Asia. We show that AsteObp1 and Anopheles gambiae Obp1 (AgamObp1) are orthologues. These two genes share similar coding sequences and conserved noncoding sequences (CNSs) that may be involved in their regulation. Transcript of AsteObp1 was observed in larvae and reached a relatively high level in late pupae. Quantitative real-time PCR on female adult chemosensory tissues showed ∼900-fold higher expression of AsteObp1 in antennae than in maxillary palp and proboscis. The amount of AsteObp1 in female legs was approximately 15-fold lower than that of maxillary palp and proboscis. The level of AsteObp1 transcript was seven and 85-fold higher in females than in males in the antennae, and maxillary palp and proboscis, respectively. Moreover, the AsteObp1 level was reduced by approximately 20-fold in maxillary palp and proboscis 24 h after a bloodmeal. Our results indicate that AsteObp1 is likely to function in the female olfactory response and may also be involved in blood-feeding behaviour.