A new parathyroid hormone (PTH)-like endocrine system has been identified in mammals and fishes consisting of the PTH type-2 receptor (PTH2R) and tuberoinfundibular peptide 39 (TIP39). Although the mammalian PTH2R-TIP39 system is involved in nociception and pituitary regulation, the function(s) of this system in fishes is undetermined. Using degenerate primers based on conserved zebrafish and fugu TIP39 nucleotide sequences, 3'-RACE reactions isolated the coding region of a putative TIP38 cDNA in the Nile tilapia (Oreochromis niloticus). Tilapia-specific primers were subsequently used in 5'-RACE reactions to isolate the remaining portion of the transcript. The cDNA encoding O. niloticus TIP (OnTIP38) was determined to yield a 38 amino acid secreted hormone. A second tilapia TIP38 cDNA was isolated from the euryhaline Mozambique tilapia (OmTIP38). Except for the 39th residue, both tilapia cDNA sequences showed significant identity to human, bovine, murine, fugu, and zebrafish TIP39. To determine the tissue-specific expression of OnTIP38 and OmTIP38, real-time quantitative RT-PCR (rQRT-PCR) was performed on skin, gill, kidney, testis, heart, and brain. In freshwater (FW)-acclimated Nile tilapia, OnTIP38 showed highest levels of expression in kidney and lowest levels in skin and gill. In Mozambique tilapia tissues, expression of OmTIP38 and G6Pase (glucose-6 phosphatase) were higher in salt water (SW)-acclimated fish than in FW-acclimated fish. G6Pase expression, and not OmTIP38, showed significant differences among various tissues in FW- and SW-acclimated fish. Results of the present study clearly indicate that the TIP38/39-PTH2R system shows considerable conservation in sequence identity and tissue-specific expression in mammals and fishes.