A down-regulated cDNA fragment was isolated using differential display PCR between ovulatory and post-ovulatory brook trout ovarian tissue, Using this cDNA as a probe, a full-length cDNA of 538 bp was obtained by screening an ovarian cDNA library. The cDNA isolated presumably encodes for a 101 amino acid protein that is similar in sequence to a group of calcium binding proteins called S100 proteins. Within the S100 family, the differentially expressed trout protein is more similar to S100 Al and A10 proteins. A comparison of the trout S100 protein with mammalian S100 Al proteins and a S100 protein previously isolated from the leach suggests that the trout S100 protein could be a new member of the S100 family. On Northern blots of ovarian tissue, the trout S100 cDNA hybridized with a 550 bp transcript which appeared to progressively decrease throughout ovulation and was significantly down-regulated by 48 h post-ovulation. A transcript was also detected in several other tissues such as the heart, skin, spleen, gills, intestine, stomach and testis. Using in situ hybridization the trout S100 mRNA was strongly detected in the granulosa cells of the ovulated follicle, These results suggest that a member of the S100 protein family may play a role in the trout ovary during the periovulatory period.