The stable isotope ratios of carbon (δ 13 C) and nitrogen (δ 15 N) of the muscle, ctenidia and viscera of the Sydney rock oyster, Saccostrea glomerata, showed the dilution and assimilation of tertiary treated sewage along an estuarine gradient. The enriched 15 N values of oyster ctenidia and viscera from within 50m of the sewage outfall indicated the use of 15 N-enriched tertiary treated sewage effluent (16±2.3‰) as a nutrient source. The effect of sewage nitrogen on oyster δ 15 N was localised, with oysters 5km upstream and downstream of the outfall not significantly enriched. Viscera δ 15 N was most sensitive to sewage nutrients and δ 13 C significantly defined an ocean-to-estuarine gradient. High variance in isotope ratios of viscera compromised its use as an indicator of anthropogenic nutrients, and this also reduced the utility of whole-body stable isotope ratios. Ctenidia was the most useful indicator tissue of sewage discharge at the scale of this study, being consistently and significantly enriched in δ 15 N close to the sewage outfall and δ 13 C clearly defined an estuarine gradient with less internal variability than viscera. Muscle δ 15 N was least sensitive to sewage effluent and showed the least variability, making it more suited to investigations of anthropogenic nutrient enrichment over larger spatio-temporal scales.