Stanniocalcin (STC) is a homodimeric glycoprotein hormone implicated in calcium and phosphate regulation in both teleost fish and mammals. In the present study, immunostaining with salmon STC antiserum demonstrated that STC cells were localized in both the corpuscles of Stannius (CS) and in specific cells of the distal renal tubules of the silver arawana, Osteoglossum bicirrhosum, an ancient ray-finned fish (actinoptergian) and basal teleost (Order Osteoglossiforme). The morphology of these STC-immunoreactive kidney cells was similar to renal 'chloride' (mitochondrial-rich) cells. The immunoreactive renal cells were present in two of three other osteoglossiformes and absent in the gar, a nonteleost actinopterygian and the eel, another basal teleost. The arawana STC cDNA encodes a prehormone of 249 amino acids (aa) with a signal peptide of 31 aa and a mature protein of 218 aa. The deduced aa sequence of arawana STC shows 54 -66% identity with other teleost STCs and 51 -52% identity with mammalian STC-1. The deduced aa sequence of arawana STC contains ten cysteines, compared with 11 in teleost STC and in mammalian STC-1. The cysteine substitution occurs at the site of inter-monomeric disulfide linkage. Western blot analysis revealed a single 21 kDa band under non-reducing conditions, and a single band of 25 kDa under reducing conditions. These data indicate that arawana STC exists as a monomeric peptide. Northern blot analysis detected a 3.3 kb STC mRNA confined to the CS, with no hybridization signal in either the remaining kidney or in gut, muscle, brain and heart. The significance of the STC signal in cells of the renal tubules of arawana and two other Osteoglossiforme species requires further investigation. This is the first report of a monomeric form of STC in any vertebrate and the first evidence of STC in renal tubules of adult fish.