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Abstract: The Tribolium castaneum genome sequence reveals a large number of odorant receptor (Or) genes compared to those found in other insects whose olfactory genomes have been studied—341 Or genes and pseudogenes, encoding 259 intact odorant receptor proteins. An RT-PCR study of larvae and adults revealed that only 145 (64%) of 233 genes with successful genomic DNA amplifications were expressed. No expression of the other 87 genes was detected at any age, suggesting either that these genes are not expressed in this particular strain, or that they are induced only in certain environmental or developmental conditions. TcOR1, the ortholog of the Drosophila Or83b (DmOr83b) gene, which is required for the function of olfactory receptor proteins in Drosophila, was expressed in extracts from adult and larval heads and in extracts from adult bodies. Expression of 41 TcOr genes was detected in extracts from larval head tissue and 111 in extracts from adult head tissue (both figures exclude TcOr1). Twenty-eight TcOrs were detected only in adult bodies. Beetle pupae were injected with TcOr1 dsRNA; unlike sham-injected and control beetles, these knock-down beetles showed no significant response to the Tribolium aggregation pheromone, supporting the hypothesis that TcOr1 plays a similar decisive role in olfaction to DmOr83b. The substantial number of Ors poses the question of why Tribolium has such a large olfactory receptor repertoire, and underlines the need for more studies of the natural history of this species. [Copyright &y& Elsevier]