The arginine vasopressin-like peptide (AVP-like or AVPL) in insects was originally described in the locust Locusta migratoria (Linnaeus) where it was found to have diuretic activity. Interestingly, previous searches of the Drosophila melanogaster (Meigen) genome and Anopheles gambiae genome for the AVP-like sequence has been unsuccessful, suggesting that this gene has been lost in higher dipteran insects. By mining the whole genome sequence of Tribolium castaneum (Herbst), we were able to identify a putative avp-like gene encoding both AVP-like peptide and neurophysin. Likewise, a putative AVP-like receptor, which is a novel G protein-coupled receptor, was identified in T. castaneum (Herbst) by BLAST of the genome sequence followed by phylogenic analysis. The avpl gene contains an open reading frame encoding 146 amino acids, and the receptor gene encodes a protein of 383 amino acids. Immunohistochemistry using an anti-AVP antibody revealed a pair of neurosecretory cells on the ventral surface of the subesophageal ganglion which bifurcates and projects throughout the CNS to each ganglion, especially to the terminal abdominal ganglia. In situ hybridization confirmed the expression of avpl in the same pair of neurosecratory cells. Authentic ligand-receptor interaction was found in a heterologous expression of the AVP-like receptor (EC50 < 1nM). The function of the AVP-like peptide on diuresis was investigated by using a highly sensitive humidity chamber assay. Strong diuretic activity was found upon injection of the AVP-like peptide in a dose dependent manner.