|In this paper, we present an evaluation of genotoxic responses in marine diatom, Chaetoceros tenuissimus, isolated from Kandla Creek (lat 23.03° N, long 70.22° E), Gujarat, India, in terms of impairment of DNA integrity as a function of their exposure to elevated levels of mercury (Hg) under laboratory conditions. DNA integrity in C. tenuissimus was determined by partial alkaline unwinding assay. To our knowledge, this is the first such genotoxicity study to be conducted on marine diatom cultures towards understanding the relationship between Hg toxicity and DNA damage. Furthermore, we studied the impact of Hg on the growth of C. tenuissimus as a function of their exposure to enhanced levels of Hg in terms of decreasing chlorophyll a (chl a) concentrations. The data show the genotoxic effect of Hg on the growth of C. tenuissimus as well as DNA integrity to a great extent. Based on the results of our investigations, it is suggested that C. tenuissimus can be used as sentinel species for bio-monitoring of pollution due to genotoxic contaminants.