An up-regulated cDNA fragment was isolated using a differential display polymerase chain reaction between ovulatory and postovulatory brook trout ovarian tissues. Using this fragment as a probe, a full-length cDNA of 1783 base pairs was obtained from an ovarian cDNA library. The cDNA presumably codes for a 383-amino acid protein with strong sequence similarity to an aspartic protease, progastricsin (EC 3.4.23.3), also known as pepsinogen C. On Northern blots of ovarian tissue, the trout progastricsin cDNA hybridized with a 1.8-kilobase transcript that was strongly up-regulated 4-6 days after ovulation. Of all other tissues tested, a transcript was only detected in the stomach. A recombinant trout progastricsin protein was produced and used to raise an antibody. On Western blots of ovarian tissue, the progastricsin antibody recognized a single 39-kDa protein that was present in the ovary only following ovulation. On Western blots of coelomic fluid, the 39-kDa protein was strongly detected 4-10 days after ovulation. The trout progastricsin was immunocytochemically localized to the granulosa cells of postovulatory follicles, suggesting that it is released from this tissue into the coelomic fluid following ovulation. Progastricsin has been found in the stomach, prostate, seminal vesicle, seminal fluid, and pancreas of vertebrates; however, this is the first report of a progastricsin in an animal ovary.