This study investigated the leaf anatomy of Odontonema strictum (Nees) O. Kuntze (Acanthaceae), with emphasis on cystoliths, which are very abundant in this species. Leaves fully expanded from five mature individuals were collected in the teaching garden of the “Setor de Ciências Biológicas” (UFPR) and fixed in FAA 70. The cystoliths were located in the epidermal cells of the adaxial face (litocysths) and also among the collenchyma cells of the midrib and petiole. The cystoliths were cylindrical, long, with several small projections. They sometimes occurred in pairs and varied greatly in size, with an average length of 163.4±68.8 µm and a density of 23±9.97 cystoliths.mm-2 (n=10). The foliar lamina presented a uniseriate epidermis, dorsiventral mesophyll, and palisade parenchyma, which was continuous along the lamina. In the midrib, arc-shaped vascular veins were surrounded by fundamental parenchyma. Glandular trichomes occurred on both faces of the leaf, while non-glandular trichomes and diacytic stomata were present only on the abaxial face. The great number and size of the cystoliths observed could be related to defenses against herbivory and to mechanisms of removal of excess calcium absorbed from the soil.