The ultrastructure of the ovarian follicle and the levels of the ovarian 11-dehydrocorticosterone (11-DHC), estradiol-17 beta (E-2). 17 alpha-hydroxyprogesterone (17 alpha-OHP), and progesterone (P) were studied during cortical alveoli, vitellogenesis and oocyte maturation in Chalcalburnus tarichi. The microvilli began to form on the oocyte surface during cortical alveoli phase and during vitellogenesis, came into contact in the pore canals of the zona radiata with microvilli formed from granulosa cells. While the special thecal cells posses specific organelles, which are characteristic of steroid-producing cells, namely, mitochondria with tubular cristae, smooth endoplasmic reticulum and lipid droplets, the granulosa cells contain organelles typical for protein-secreting cells. These findings suggest that during vitellogenesis the special thecal cells are the sites of steroid synthesis in the C. tarichi ovary.