Tetrodotoxin (TTX) extracted and purified from puffer fish Arothron nigropunctatus was tested for genotoxicity employing the root meristem cells of Allium cepa as the assay system. The genotoxicity endpoints investigated were mitotic index (MI), meta-anaphases with spindle aberrations, interphases with micronuclei (MNC) and sister chromatid exchanges (SCE) in metaphase chromosomes. The results demonstrated that TTX inhibited mitosis at concentrations of ≤ 30 μM as evident by the fall of MI, but failed to induce MNC at significant levels at any of the concentrations tested (10-100 μM). TTX was thus proved to be neither clastogenic nor aneugenic in the present study. It was, however, noteworthy that TTX at far lower concentrations, 0.1-5.0 μM, significantly enhanced the frequencies of SCE which indicated possible interference of the toxin in DNA replication and repair.