The present study was performed to investigate, ultrastructurally the effect of heavy metals on the regenerating tail fin of the teleost fish, Oreochromis niloticus, revealed that, in the tail fins of the specimens treated with lead (Pb) at 0.005 mg/l, at the fifth day postamputation, the blastema cells are displayed shrinkage nuclei, much chromatin, pyknotic mitochondria and vacuolated cytoplasm, whereas in the specimens treated with 0.025 mg/l, the blastema cells showed multinucleoli, different size and shape of mitochondria and swollen rough endoplasmic reticulum. In the tail fins of the specimens treated with cadmium (Cd) at 0.005 mg/l, the nuclei of the blastema cells showed little chromatin, also, a blood vessel with abnormal red blood cells was recognized in the neighborhood layer of the blastema cells, whereas in the specimens treated with 0.025 mg/l the nuclei of the blastema cells are arranged adjacent to the inner membrane of the nuclear envelope. The cytoplasm of these cells displayed swollen rough endoplasmic reticulum, with the transformation of their cisternae into circular arrays, vacuolated cytoplasm, pyknotic mitochondria and suppression of differentiation. In the tail fins of the specimens treated with mercury (Hg) at 0.0006 mg/l, the blastema cells showed dense scattered lysosomes, pyknotic mitochondria, progressive damage in the cytoplasmic organelles leaving large vacuolated space and complete autolysis of cells. In the tail fins of the specimens treated with Pb at 0.005 and 0.025 mg/l and Cd at 0.005 and 0.025 mg/l, at the seventh day postamputation, the lepidotrichia forming cells have dense patches of irregular lysosomes, swollen and degranulated rough endoplasmic reticulum, pyknotic mitochondria and vacuolated cytoplasm. In the tail fins of the specimens treated with Hg at 0.0006 mg/l, showed deformation of the lepidotrichia forming cells, there was an invasion of leucocytes and