Genotoxic effects of an aqueous extract of Nicotiana tabacum, a variety commonly used in India for chewing purposes, were analysed on CHO cells utilizing two different cytogenetic end-points, namely, chromosome aberration frequency and sister chromatid exchange frequency. Statistically significant elevations in the values of both the markers clearly indicated chromosome damaging effects of the extract. Elevations in chromosome aberration and sister chromatid exchange frequencies are suggestive of intrastrand and interstrand DNA cross-links following exposure to tobacco. The effects observed following treatment with low dose for longer duration are of relevance to the condition of the oral mucosa of the chronic smokeless tobacco users.