The effect of the oral administration of aqueous suspension of Nigella sativa (50 mg/kg b.wt.) against chromosomal aberrations and ultrastructural changes of the bone marrow cells in mice treated with carbon tetrachloride CCl4 was studied. CCl4 was administered in two dose levels equivalent to1⁄4 (1.9 ml/kg b.wt.) and 1⁄2 (3.8 ml/kg b.wt.) of the oral LD50 in mice. The data indicated a significant dose-dependent decrease in the mitotic activity of bone marrow cells in animals treated with CCl4. Also a significant dose-dependent increase in the number of bone marrow cells with different types of chromosomal aberrations was recorded in these animals. Ultrastructural changes were also dose-dependent including both nucleus and cytoplasm of erythroid and myeloid elements of the bone marrow cells. Treatment of the animals with N. sativa improved both genotoxicity and ultrastructural changes induced by the two dose levels of CCl4.