Titanium dioxide (TiO 2 )is one of the most widely used nanoparticles, and aquatic organisms are especially exposed to it. To examine reproductive toxicology, zebrafish were exposed to different concentrations (1, 2 and 4 mg/L) of TiO 2 nanoparticles. The ultrastructure of the theca cell, zona radiata structure and follicular epithelium were examined in detail by transmission electron microscopy (TEM). No abnormalities were observed in the control group; however, degeneration of pore and microvilli structures of the zona radiata, vacuolization in the ooplasm, mitochondrial swelling and mitotic catastrophe (the mechanism for eliminating mitosis-incompetent cells in eukaryotes) were detected in the exposure groups. These results indicate that TiO 2 nanoparticle exposure causes paraptotic-type cell death in zebrafish oocytes, follicular and theca cells. In light of the observed histopathological changes, it was concluded that TiO 2 exposure inhibited oogenesis and the reproductive capability in zebrafish. https://doi.org/10.2298/ABS170303035A Received: March 3, 2017; Revised: August 8, 2017; Accepted: September 20, 2017; Published online: October 3, 2017 How to cite this article: Akbulut C, Kotil T, Yon DN. Ultrastructural evaluation of oocyte envelopes of zebrafish ( Danio Rerio ) (Hamilton, 1822) after TiO2 nanoparticle exposure. Arch Biol Sci. 2018;70(1):159-65.