Abstract : The endobenthic species Ophelia bicornis (Savigny, 1818) plays a crucial role in evaluating the structure and functioning of estuarine ecosystems. The polychaete worm is notable for its assessment of physiological tolerance to various environmental factors, with particular emphasis on its ability to withstand sediment-bound contaminants. In the ecological assessment, Ophelia bicornis lives in a rich organic-matter environment and is used as an indicator species of pollution. This research aims to evaluate the impact of a xenobiotic, "cadmium," considered one of the most toxic heavy metals, on Ophelia bicornis. Mortality was assessed, and median lethal concentrations (LC10 ; LC50) were calculated. Cadmium was added to the rearing seawater of O. bicornis at four concentrations (30- 45- 75- 120 mg/L) to determine the different lethal concentrations. The LC50, and LC10 values for 48, 72 and 96-h with 95% confidence limits were estimated by the probit method and were found as respectively 139.31 (115.13- 168.56), 77.26 (69.60- 85.75), 59.70 (40.89-87.16). In contrast, LC10 values were 31.11 (25.71-37.64), 25.82 (23.26-28.66), 22.80 (21.11-24.62) respectively. LC50 and LC10 decreased with an increase in mean exposure times. Furthermore, increased metal concentrations and exposure times were associated with increasing mortality rates among O. bicornis. The established lethal Concentration values could be used as key indicators in toxicity tests to demonstrate the effects of toxic pollutants, particularly for ecotoxicological evaluation of marine ecosystems in risk assessments. These results contribute to a better understanding of the environmental risks posed by cadmium and help refine the use of O. bicornis in ecotoxicological studies.