Isodon coetsa (Buch.-Ham. ex D. Don) Kudo (Lamiaceae) belongs to a genus typified for the presence of bioactive ent-kaurane diterpenoids and have been used in traditional Chinese folk medicine as antibacterial, antiinflammatory and antitumour agents for a long time. The aqueous leaf extracts were tested for cytotoxicity screening using Allium cepa assay due to its biological activity. Root tips were treated with different concentrations of the extract viz., 0.005 %, 0.01 %, 0.05 %, 0.1 % for varying time durations (½ h, 1 h, 2 h, 3 h). Increase in mitotic index was observed in all treatments. High frequency of aberrations viz., C-metaphase, polyploidy, cytostasis and vagrant chromosomes were observed in treated root tips, which revealed the C-mitotic effect in the extract. The plant extracts show severe as well as partial C-mitotic activity which was time and concentration independent. C-mitotic activity was found to be prominent during 1 h treatment at all concentrations which may be due to the cytostatic effect. Higher duration treatments at all concentration revealed a decrease probably due to the cytotoxic effect during prolonged exposure. Induction of polyploidy was observed at all treatments, which do not show any correlation either with concentration or time, probably due to the metabolic imbalance of the treated cells. Thus, it is concluded that Isodon coetsa leaf extracts possess C-mitotic activity which can be the effect of the presence of a colchicine like compound in it.