Estrogens are steroid hormones involved in the control of important reproduction-related processes including sexual differentiation and maturation. Careful spatial and temporal balance of estrogens in the body is crucial for proper functioning. At the final step of estrogen biosynthesis, cytochrome P450 aromatase, encoded by the cyp19 gene, converts androgens into estrogens. Modulation of aromatase CYP19 expression and function can dramatically alter the rate of estrogen production, disturbing the local and systemic levels of estrogens. This activity has been shown to be sensitive to many endocrine disruptive compounds (EDC) released in the environment and thus could be used as a biomarker to screen for EDC effects. Teleosts are becoming increasingly important indicators of environmental health, and roach, Rutilus rutilus, was suggested as a sensitive model organism to assess the impact of xenobiotic in freshwater. A study has been undertaken to characterise both gene expression and activity of aromatase in the brain and gonad. Subsequently, seasonal variations, incidence of physical factors, biological factors (sex, age), presence of chemicals and association with pathological features (parasites, intersex) have been studied