Genotoxic effects of Cadmium on phytoplankton Chaetoceros tenuissimus have been evaluated using DNA damage by comet assay. Cadmium concentrations ranging from 2.4 to 10 mg/L were used to evaluate the effects. Results showed that as the concentration of Cd increased growth of the diatom decreased. Alkaline single-cell gel electrophoresis (Comet assay) method, which is highly sensitive in detection of DNA damage in eukaryotic cells, was used to observe genomic changes in marine diatom cells. DNA damage was measured as percent number of comets and normal cells. 65% cells were found to be damaged at 10 mg/L concentration of Cd as compared to 23% in 2.4 mg/L and only 5% in controls. More than 50% apoptic cells were observed on 8th day at 10mg/L and 12th day at 7.5 mg/L concentrations. At lower Cd concentrations (4.5 mg/L and below) the damage was below 30% till the last day. This suggested that higher Cd levels have early damaging effects on cell nuclear material and that % injury increases with advancement of exposure period. One advantage of use of C. tenuissimus is the ease with which it can be cultured in a defined medium. Chaetoceros tenuissimus diatom can be used as an in vivo model for ecogenotoxicity assessment using the Comet assay.