Ionizing radiation is known to induce genetic damage in diverse groups of organisms. Under accidentalsituations, large quantities of radioactive elements get released into the environment and radiation emit-ted from these radionuclides may adversely affect both the man and the non-human biota. The presentstudy is aimed (a) to know the genotoxic effect of gamma radiation on aquatic fauna employing twospecies of selected bivalves, (b) to evaluate the possible use of ‘Comet assay’ for detecting genetic dam-age in haemocytes of bivalves as a biomarker for environmental biomonitoring and also (c) to comparethe relative sensitivity of two species of bivalves viz. Paphia malabarica and Meretrix casta to gamma radiation. The comet assays was optimized and validated using different concentrations (18, 32 and 56 mg/L)of ethyl methanesulfonate (EMS), a direct-acting reference genotoxic agent, to which the bivalves wereexposed for various times (24, 48 and 72 h). Bivalves were irradiated (single acute exposure) with 5 dif-ferent doses (viz. 2, 4, 6, 8 and 10 Gy) of gamma radiation and their genotoxic effects on the haemocyteswere studied using the comet assay. Haemolymph was collected from the adductor muscle at 24, 48and 72 h of both EMS-exposed and irradiated bivalves and comet assay was carried out using standardprotocol. A significant increase in DNA damage was observed as indicated by an increase in % tail DNAdamage at different concentrations of EMS and all the doses of gamma radiation as compared to controlsin both bivalve species. This showed a dose-dependent increase of genetic damage induced in bivalvesby EMS as well as gamma radiation. Further, the highest DNA damage was observed at 24 h. The damagegradually decreased with time, i.e. was smaller at 48 and 72 h than at 24 h post irradiation in both speciesof bivalves. This may indicate repair of the damaged DNA and/or loss of heavily damaged cells as the postirradiation time advanced. The present study reveals that gamma radiation induces single strand breaksin DNA as measured by alkaline comet assay in bivalves and comet assay serves as a sensitive and rapidmethod to detect genotoxicity of gamma radiation. This study further indicates that both M. casta and P.malabarica exhibit almost identical sensitivity to gamma radiation as measured by DNA damage.