The stanniocalcin (STC) gene was recently found to be widely expressed in fish. In this study, we have characterized ovarian STC in the rainbow trout (Oncorhynchus mykiss) and cloned the ovarian cDNA. The STC gene expression was highest in early stage oocytes and diminished progressively as oocytes developed. At the cellular level, ovarian STC gene expression was most abundant in the ooplasm of early stage oocytes, but it was also weakly evident in the theca layer, interstitial cells, and vitellogenic oocytes. The STC protein was distributed in a pattern similar to that of gene expression but was also apparent in glycoprotein vesicles, nuclei, multivesicular bodies, and follicles undergoing atresia. Cloned cDNAs obtained from the corpuscles of Stannius (CS) and ovarian transcripts were nearly identical. However, Western blotting of the partially purified proteins revealed that ovarian STC was larger than CS STC. Further analysis revealed that ovarian STC had a much larger N-linked carbohydrate moiety (∼12 kDa) compared to CS STC (∼7 kDa), indicating that the two hormones were differentially posttranslationally modified. To our knowledge, this is the first characterization of STC gene expression, cDNA, and protein distribution in the piscine ovary and the first evidence for any difference between alternative sources of the hormone in any species.