Follicle rupture during ovulation is a well-regulated biological process of extracellular matrix degradation in the vertebrate ovary. Although proteolytic enzymes responsible for the rupture have recently been identified in the medaka, Oryzias latipes, the lack of knowledge about the ovarian expression and distribution of extracellular matrix components in lower vertebrates prevents the understanding of this process’s molecular mechanism. To approach the problem, we cloned a cDNA coding for the medaka collagen type-I α1 chain and examined its mRNA expression in the fish ovary. The deduced amino acid sequence of the collagen type-I α1 chain was homologous to those of the proteins from other vertebrate species. The α1 chain mRNA was expressed in various tissues of the adult fish. In the ovary sections of mature female fish, this mRNA was detected in a line surrounding ovarian follicles of all sizes. A comparison with the distribution of gelatinase B mRNA in follicles that had just ovulated indicated that the collagen type-I α1 gene is expressed in the theca cells. The current results strongly suggest that collagen type I is synthesized by theca cells and is localized in the same cell layer of the follicles.