<p>Ovarian development of fishes involves significant functional and structural changes during the process of mature egg formation. In this study, we report the folliculogenesis and follicular growth in the endangered species <i>Brycon opalinus</i> as well as the dynamics of the proteins Foxl2, Col4 and MMP-9 during ovarian maturation. Seventeen females were collected in the Doce River basin and their ovaries were subjected to histological techniques, electron microscopy, immunohistochemistry and Western blotting. In ovaries of <i>B. opalinus,</i> isolated oogonia and oogonia grouped in niches were identified, both surrounded by prefollicle cells. Primary growth initiated when the ovarian follicle was formed by the oocyte and follicle cells, in addition to the formation of zona radiata. Secondary growth was characterized by the incorporation of yolk into the oocyte and development of the zona radiata and follicle cells. Proportion of oogonia, perinucleolar follicles, and ovarian stroma was higher during resting stage, while previtellogenic and vitellogenic follicles were more frequent in the early and advanced maturation stages. Foxl2 was expressed mainly in prefollicle cells in oogonia and follicle cells in young follicles during reproductive resting. Col4 was expressed in basement membrane and extracellular matrix, primarily during resting and early maturation of ovaries. MMP-9 showed discrete labeling in ovarian matrix, but theca cells and zona radiata of secondary growth follicles, as well as granulocytes in matrix, showed significant immunolabeling during advanced maturation. We conclude that during ovarian development of <i>B. opalinus</i> there is intense participation and interaction of Foxl2, Col4 and MMP-9 with matrix components, somatic and germ cells for formation and follicular growth.</p>