Biting flies are economically important blood-feeding pests of medical and veterinary significance. Chemosensory-based biting fly behaviours, such as host/nutrient source localization and ovipositional site selection, are intriguing targets for the development of supplemental control strategies. In an effort to expand our understanding of biting fly chemosensory pathways, transcripts encoding the highly conserved insect odorant co-receptor (Orco) were isolated from two representative biting fly species, the stable fly (Scal\Orco) and the horn fly (Hirr\Orco). Orco forms a complex with an odour-specific odorant receptor to form an odour-gated ion channel. The biting fly transcripts were predicted to encode proteins with 87–94% amino acid similarity to published insect Orco sequences and were detected in various immature stages as well as in adult structures associated with olfaction, i.e. the antennae and maxillary palps, and gustation, i.e. the proboscis. Further, the relevant proteins were immunolocalized to specific antennal sensilla using anti-serum raised against a peptide sequence conserved between the two fly species. Results from the present study provide a basis for functional evaluation of repellent/attractant effects on as yet uncharacterized stable fly and horn fly conventional odorant receptors.