We have used the CapFinder technology, without the library construction step, to amplify and clone full-length cDNAs expressed in the labial palps (CO2-sensing organs) of the moth Cactoblastis cactorum. The validity of our approach is exemplified by the sequence analysis of a 597-bp cDNA clone, designated CLP-1, that contains a 390-bp open reading frame (ORF) flanked by motifs characteristic to a full-length cDNA. The ORF in CLP-1 encodes a predicted polypeptide that is 47% identical to a novel protein, OS-D, found exclusively in the olfactory antennal segment of Drosophila melanogaster. Both CLP-1 and OS-D have primary structures that do not bear sequence similarity to any previously characterised proteins including odorant-binding proteins (OBPs) in vertebrates and pheromone-binding proteins (PBPs) in moths. Although they share features common to OBPs and PBPs, such as the presence of signal peptides and cysteine motifs, they clearly belong to a distinct class of olfactory proteins that appear to be unique to insects. The relative abundance of the CLP-1 message in the labial palps of females leads to the suggestion that this protein is involved in the CO2-sensing cascade. Our results suggest that the experimental procedure can be used as an alternative, rapid method to identify genes expressed in a particular organ, or tissue, especially in situations when the amount of available tissue is a limiting factor.